What does wattage of amplifier corresponds to? Does more wattage means more volume or high gain?
The rated output power of a guitar amp doesn't really tell you much. It has nothing to do with how much distortion you can get from an amp. It is only slightly related to how loud an amp is. Some amps with a lower output power can get louder than other amps with higher output power.
The only thing the output power really tells you is how far you can make what size speakers move. So an amp with more power can power more speakers and make them move farther. That seems like it's the same as being louder, but loudness is actually pretty complicated and is not just about moving a lot of air.