#1
I'm here looking at bass amps out of boredom and I'm looking at an amp(fender rumble100/210) that says 100 watts at 4ohms. My question is what does ohms mean(the title is just an example).
#2
It means that when the speakers are connected is such a way that the speakers total 4 ohms (one 4-ohm cabinet or two 8-ohm cabinets, for example) it puts out 100watts of power.

If that amp has a speaker out, it's probably running at 8 ohms, meaning you can connect another 8 ohm cabinet to get the full power.