#1
what does "clean" mean?

Clarity, punch, repsonsive EQ, amount of filtered sine wave, what?

what do people mean when they say something has good cleans?
#2
It means that the low-THD range of sounds it produces is pleasing to the ear.
#4
Total Harmonic Distortion.

You know what cleans are, you know what the adjective "nice" means, I'm not sure you have a question.
#5
re-educate me on what cleans are, this way if i start from scratch, i wont post another million threads...
#6
Cleans are when you don't have significant signal clipping. Hence the THD comment, THD is a way of measuring signal clipping/distortion.