There should be little pop-up tips next to each field in FreePBX so, for example, if you hover your mouse over the little dot next to Ring Strategy, it’ll give a brief description of those.
firstavailable only rings the first available agent instead of ringing all agents in the ring group. firstnotonphone appears to be the same, except it ignores call waiting. firstavailable will ring a phone that is in use if call waiting is enabled on it while firstnotonphone will skip that phone.