I want to make mods and I am just learning how to, one of the things I havent touched on is armor and I was wondering if anyone knew how that worked. I have a vague idea about the mass of the armor changing how heavy a bullet it can block, but i am not sure.

You can add armor plates to actors/crabs/craft/weapons by using attachables.
Look at the coalition heavy and dummy dreadnought code for reference.

Strength of the armor is depending on the material it is made of.
The stronger the material, the less chances the armor is penetrated (entry wound).
Strong materials like "Mega Metal" will cause low velocity/low sharpness particles to bounce off it, causing no damage.
High velocity/high sharpness particles can penetrate armor all the way (entry wound + exit wound), hitting your actor.

Again, see coalition heavy and dummy dreadnought for better understanding.
Also you can check my MDC mod, I used plenty of attachable armor on actors, craft, crabs, turrets - basically everything.

Also something to note is that unlike the health bar, armor works on how many times it's hit - unless you set the gibimpulselimit low, in which case a strong bullet or a firm gust of wind will gib your armor. I generally set the impulse limit high enough so no conventional bullet can take it off in one shot. The gibwoundlimit is the amount of times it can be shot, by a bullet of any strength (again, if you set the impulselimit high enough), before it breaks.

Ok, so it has nthing to do with the mass of the attachable?

Absolutely nothing.
It has everything to do with the "StructuralIntegrity" variable of the material you are using.

Best way to learn how everything works is to mess around with variables and seeing how they behave ingame.
It's all trial and error. Worst case scenario is your game crashing without telling you why.