ohms are resistance.
Basically the lower the ohms, the louder (typically) but also it becomes easier to fry stuff.
My Bass Guitar amp runs 1600watts mono bridged at 4ohms or ??? bridged at 8 ohms. I run the ??? bridged because I run two cabinets each with 4 10" speakers and an adjustable horn in each. Each Cabinet is rated at 8ohms and have a peak power output of 1000watts each.
All together it is loud, but not the loudest out there even though it is currently the most powerful bass guitar amplifier available. It is the clarity at all sound levels, I can crank it up and still have good sounding lines.
I used to know a lot about this stuff but I quit playing for nearly a year after playing for 4 years striaght. and im just getting back into it again.
For those who want to know my setup Ill post it in the off topic section.