RDM = Random Death Match. Basically Killing someone without a valid reason i.e: mugging, raid, counter
RDA = RanDom Arrest. Everyone prefers to say RDA to RA, just makes more sense but like above it's when you're arrested without a reason.
NLR = New Life Rule. Not all servers enforce this but the basic premise is that when you die you forget everything for 10 minutes. You can not go back to your base if you were being raided, you cannot revenge kill someone. You simply do not remember for 10 minutes. (To be perfectly honest though most people don't really follow this to the T)