A hotkey is a key on the keyboard which activates something within a system. Specific to World of Warcraft, a hotkey activates abilities or skills, opens panes, begins or ends automatic actions, or, in general, performs whatever it has been scripted to do.
'Binding' and 'keybinding' are equivalent terms.
How to Edit Hotkeys
Players can edit their hotkeys via the options menu ingame. The pane lists every hotkey currently made accessible by the UI, including those which AddOns have created.
Whenever the keybindings of an account get edited, they synchronize themselves with the server. This makes a re-installation of World of Warcraft much easier.
However: Sometimes this isn't prefered. (Maybe to play with different computers and different keybindings.) So it can also be disabled ingame. (To enable it, just replace the 0 with a 1)
/console synchronizeConfig 0
/console synchronizeBindings 0