Contextual links are supplied by modules, to give you quick access to tasks associated with regions of pages on your site. For instance, if you have a custom menu block displayed in a sidebar of your site, the Blocks and Menus modules will supply links to configure the block and edit the menu. The Contextual links module collects these links into a list for display by your theme, and also adds JavaScript code to the page to hide the links initially, and display them when your mouse hovers over the block.