Dock
From MoodleDocs
Template:Moodle 2.0 The dock in Moodle 2.0 is an area where blocks can be moved to and from this area with the block to dock and dock to block icons.
The "docked" blocks appear as horizontal tabs along the left margin attached to the page.
The docked block can expand into a menu with a click on the name link.
Clicking on the dock icon when a block is in the docked position, will move all blocks back to their original position.