The Contributors tab shows you:

• in the upper part, the graph of all contributions in the repository,
• in the lower part, all repository commiters with their personal activity graphs and a total of lines of code added (green) and deleted (red) by commiter. 

In the Show menu, you can choose whether commits, code line additions or deletions should be displayed in the general graph. You can drag across the graph to show contributor analytics for a certain period of time (this feature is available as a part of the Advanced pack).

The contributors are ordered by value specified in the Show menu. For example, if Commits is chosen, contributors are ordered by a number of commits they made in the repository.

Clicking the name of a user takes you to the Contributions graph (this feature is available as a part of the Advanced pack).