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?
It means that the low-THD range of sounds it produces is pleasing to the ear.
Total Harmonic Distortion.

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