Simple English Wikipedia is a version of Wikipedia written in a simpler style and a more basic vocabulary.
According to the main Wikipedia entry on its Simple English sister,
The articles on the Simple English Wikipedia are usually shorter than their English Wikipedia counterparts, typically presenting only basic information . . . The project uses around 1,500 common English words, and is based on Basic English, an 850-word auxiliary international language created by Charles Kay Ogden in the 1920s.