To "bind" a key on ones keyboard to preform a specific task when struck. Normally preformed in console baised PC games such as Counter-Strike. This is generally used to allow the player to say entire sentences with the stroke of 1 bound key.
Player brings up his console and types... bind \ "say LoL we pwned you wOoT!".
After this whenever the player strikes his \ key his character will say "LoL we pwned you wOoT" in his chat menu.