For general information about Explorer++’s toolbars, ie. moving, resizing, customizing, etc., see the toolbars Overview.
The Drives toolbar displays all available volumes currently available on your computer. Clicking a button on the toolbar changes the address of the current tab in the Files pane to the root of that drive letter (eg. clicking C: opens C:\ in the current tab). Hovering over a drive button displays the free space and total size of the drive as a tooltip, if available (not available if the drive is not ready).
Some users may find it helpful to resize the Drives toolbar to its smallest, and dock it on the same row as the Main toolbar (right-hand edge). This frees up space, while still providing the functionality of quickly accessing every drive. When the toolbar is docked in this way, only the chevrons are visible; clicking on the chevrons opens the toolbar as a menu.
Using the middle-mouse button on a toolbar drive opens that drive in a new tab, however, docking the Drives toolbar in this way does not allow this usage on a menu item (see tip here).