A capability is permission to perform one or more types of task. Each user of a WordPress site might have some permissions but not others, depending on their role. For example, users who have the Author role usually have permission to edit their own posts (the "edit_posts" capability), but not permission to edit other users' posts (the "edit_others_posts" capability). WordPress comes with six roles and over fifty capabilities in its role-based access system. Plugins can modify the system.

Reference http://codex.wordpress.org/Glossary