Level skip glitch is a glitch that involves skipping a level by abusing the door exit button thing. I am not an Any% or 100% runner; I've ran No Level Skip and Glitchless categories, there are experts on the Discord who can help you with any questions you have

Hop on our discord and we will help you out. If you want to play as smooth as possible, download the flash player and the game from resources

PurpleBox, go to resources and download the standalone flash player as well as Cat Ninja (and the practice version if you want). You can open them in the standalone flash player and play

It means they use a flash player to run the game to reduce lag. It may provide little to no difference for some depending on your computer.

Yes, UWC is allowed in all categories except glitchless. UWC is Universal Wall Clip for those who are unaware