Binary

Wikipedia also has an article about Binary.

The binary number system uses the number two as its base. Binary numbers only use the digits 0 and 1 and are managed internally by the game (or hardware) itself. In the Pokémon glitch community, the term binary or BIN refers to the base two form of an identifier, as opposed to the base sixteen (hexadecimal or HEX) form. Practically, Binary can be useful for documenting the effects of a relatively small amount of variables, for example the value 0 can be used to denote the state 'OFF' and the value 1 can be used to denote the state 'ON'.