Mods in Fallout 4 (Ps4)?

I've been playing Fallout 4 recently and saw the Mods category at Pip-Boy, can you tell me what the mods are for, how to activate / install them and what there are for which?

You can collect them and "build" on your weapons.

A mod is for example a rifle scope or a silencer, there are many different ones for weapons and armor.