they basically NOP RPC's, RPC's are remote procedure calls, they are like functions that are called in the game in simple words, like SetPlayerPos sets the local player's position to bla coords, SetPlayerHealth, RemovePlayerWeapons etc.
NOP's stop these functions from being called, aka if you NOP SetPlayerPos, the server will try to set your pos somewhere but your character won't move an inch, SetPlayerHealth, the server tries to kill you off with custom damage but you don't take damage or die because of this being nopped. And most importantly, RemovePlayerWeapons and other weapon related, the server will REMOVE the weapons server-sided but they will still remain on the client side if you NOP BUT you will be detected for weapon hack because you don't possess these weapons anymore server-sided.
They are most useful for exploiting glitches because these functions, make it easier to trick the script into hard coded scenarios etc.