I'm not very experienced in drawing in MS Paint and I really feel like I'm going to need to know how to draw in MS Paint to draw socially acceptable graphics.
Anti-aliasing is when an art program will attempt to smooth out a line.
While this is normally a good thing, the paint bucket in paint isn't equipped to deal with this. Filling in a circle with anti-aliasing leaves these ugly white spots. because paint's paint bucket doesn't work with color tolerance, and rather fills the area of the EXACT color that you clicked on.