A "base" refers to a picture already mostly drawn, then the "artist" fills in colors and specific details to make it whatever character(s) they want it to have. People dislike them because it isn't creativity of the artist, but then putting different colors on something. Also, they tend to be poorly done (as the people who use them typically dont have the art skills to draw their own picture.)
Here is an example of a piece in which someone used a base to make:
