This is a term referring to taking a still of a movie or cartoon, and then editing it in MS Paint. It is especially common with newer/younger poopers, with most professional poopers using more advanced editing. MS Paint is frequently used as a cheap alternative to other programs such as Photoshop, since the user is too lazy/cheap to go out and buy these programs, which actually cost (gasp!) money, as opposed to MS Paint, which comes free with most versions of Windows, even though some freeware programs exist that are better than Paint.
However, there are some who use MS Paint specifically for the challenge of trying to make something good come out of it. A Youtube Pooper who mastered MS Paint with Windows Movie Maker, was IAmtheGang, who used this style for all of his videos. However, IAmTheGang never called himself a Youtube Pooper.
This tactic is considered very cheap by many, and is usually considered a lower-budget equivalent of using Adobe After Effects for a professional monster movie.