Stiffness - Pretty self explainatory, its usually better if the rear suspension is a little softer than the front, gives more traction and control

Fast bump - how fast the the suspension compresses going through bumps

Bump - how fast the suspension compresses when braking, accelerating, turning

Fast rebound - how fast the suspension decompresses after hitting bumps (goes back to normal)

Rebound - how fast it decompresses when braking, accelerating, turning

Now the important stuff!

Toe out (negative toe) - At the front wheels it will make the car take corners a little easier, especially with grip cars. At the rear wheels, its going to do the same but with a much greater effect, since thats where all the power goes. That also means, that your car is NOT going to go in a straight line when going full gas.

Toe in (positive) - it does the exact opposite. Front wheels are going to go in to turns harder. At the rear wheels though, your car will want to straighten up. Rear toe in is really helpful with drag builds, or if you cant hold your car straight.

Camber - Negative front camber will help you get more traction to the front wheels. It will not affect your handling too much, and should be mostly adjusted to the other suspension and steering settings. Positive REAR camber will help you to turn out of drifts a little easier, while negative will help you hold longer drifts.

Anti roll bar - reduces bodyroll and CAN add traction, which means more control. If there is a lot of body movement, use this. If the engine is in the front, you should have stiffer ARB on the front. Having the same stiffness isnt bad, but its usually better for the rear to be softer, because the cars rear is lighter. Also you should not make them too stiff because it can lift up your inner wheels, leading to loss of traction. IMPORTANT you can mitigate body roll with the spring ratios if you take your time, (which i recommend) but its a loooot easier to just use ARB.

Caster - Low caster will make the car smoother in turns, high caster will make the car twitchier, but its good if you want to take really fast turns. 0 caster will be the smoothest, but it will make your steering slow and sluggish. (if you play on keayboard i really recommend 0 caster, because you will be able to make adjustments a lot better in longer turns)

Ackerman - Will make your car hold speed in big angles, and it will make your drifts wider too. Its good on East Touge since that map has some really long turns, but if you add too much your car will be spinning out a lot.

Steering axis offset - Will make your front wheels hold themselves straight while drifting, but adding too much will decrease the drift angle. (this setting will not make your car spin out sooner but it makes it harder to correct it when you do) This setting is also helpful for keyboard players to be smooth. It is also helpful for drag tunes to remain straight.

Kingpin angle inclination - this will make your front wheels lose traction sooner, but you will be able to recover it easier. Really useful for reverse entries. If you add too much you will be constantly spinning out.

Steering angle - you should always have this maxed out at 65, there are 0 drawbacks to this, you will spin out later, and be able to hold larger angles.

Diameter - Big wheels will make you faster, go over stuff easier, but will slow down the turning rate of your front wheels.

Tyre width - Wider = more grip, thinner = less grip.

Tyre pressure - Lower will help your car to handle bumps a little smoother, but too low will make your wheels twitchy and less predictable. Higher will do the opposite. Depending on the car and its weight, higher or lower might be better. Tyre pressure changes will feel more obvious if you have big tyre profiles.

Adhesion - 90 is really worn, will make you be smoother but slower. It feels a little bit like driving on ice compared to the others.
100 is a good balance between everything, just normal tyres, will probably feel the most predictable since its the default.
110 is semi slicks, you can use this if you want to be faster and have more control, but you wont be as smooth.
120 is slick and has the most grip and does the same as 110 but with a greater effect, its the best for grip and drag tunes.

Wheel track - It will not affect your handling too much, you should adjust this for the fitment you want.

Tyre profile - Big profile is smoother but gives a tiny bit of a dead feeling. Best setting is between 25 and 35. Tyre width will change the profile too, so use this setting to find a good balance between your wheels. Only situation when bigger is better if you try to make a dragster or your wheels are really small.

For this, its usually the best to have maximum torque low and just use the rest to get the HP you want. Having a high rev limit will make your gears slightly longer.

Transmission is really hard to tune.

Diff locking ratio - I presonally use 70, lower will be slower and less reliable at delivering power. 100 will make your car straighten up and slow down when you try to change sides.

Final drive - lower will make your car accelerate slower but have a higher top speed. Its good to decrease this if your car is constantly redlining and you always have to speedrun through all the speeds, or you cant find a good gear.

The rest will do the same thing, but individually to every gear. Adjusting one will affect the other ones too. If you want a longer gear, just pull the slider down a little, if you want a shorter gear do the opposite. This one is really tricky because it is very car depedent. But if you mess around with it for a while you will figure it out.

Differential types:
open - no real reason to use this
spool - basically a locked diff
Viscous limited slip - its the standard type, its perfectly fine to just always use this.
Clutch Based LS - this is expert level, allows you to adjust a ton of stuff, but not too useful for the average player

Final section on this list.

Brake torque - makes you stop quicker, but its harder to make fine inputs while drifting, so its better for it to be low if you want to be smooth.

Brake bias - Lower will make your rear brakes stronger, and higher will make the front stronger. Its best to keep it a little above 50, so its really predictable, but also counteracts the weight of the engine.

ABS - turning it on will help you stop quicker but turning it off will help you be a little smoother at drifting.