A mod is basically a modification for BF2, hence the name mod. There is quite a few Mod developers for BF2 such as, Black Sand Studios, Sir Community, and others.
Modding communities Make mods for the people who play BF2 that would like to play something diffrent other than the same BF2.
They have alot of mods released and some that are still being worked on. It will not effect BF2, you'll still be able to play BF2 normally.
Personally, I'm a big mod player for BF2.
Check out the link for a list of mods:
BF2 Mods More modsIf you have any more questions, please feel free to ask.