If you're not familiar with keybinding, googling the term in concert with the name of your favorite MMO will provide plenty of examples. The long and short of it is the practice of using efficient and easy-to-reach keyboard shortcuts for all of your avatar's primary functions.
The conventional wisdom says that keybinders are generally able to react faster than those who click their UI with the mouse to fire off abilities, buffs, consumables, or what have you, and this extra speed leads to a theoretical advantage in PvP and competitive PvE situations. Exceptions exist, though, so vote for your preferred method after the cut.
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