Lead sections of Wikipedia entries include everything that comes before the table of contents, assuming one is present. (For very short articles without a table of contents, this does not quite apply.)
Because a Wikipedia article's lead section may be the only part of the entry a reader bothers with, Wikipedia's official style guide advises that the introduction serve as a concise summary for the full article.
The intro should begin with the title of the page followed by a single sentence explaining the concept. Furthermore, it should give appropriate emphasis to various aspects of the topic, and above all be easy to read. Most importantly, the lead should only include information covered later in the article.