HEX Number is short for hexadecimal number, referring to the hexadecimal number that points to the location of a piece of data (such as a Pokémon or an attack) in the game. In the Glitchologist's vocabulary, however, it is often used to mean the decimal number of a Pokémon or attack instead, which is technically incorrect. Knowing the HEX number (or, more commonly, the decimal number) of a Pokémon is important if you wish to use the Ditto Trick to find it.
Related Articles:
The Ditto Trick - HEX numbers are important because you need to know them to use this glitch.
The BIG LIST - Use this to find the HEX or decimal number of any Pokémon, attack, or item in the R/B/Y/G/S/C Pokémon games.
Equivalent Trade - Trading one glitch between R/B and Y versions produces an equivalent glitch with the same HEX number (ex. 'M and 3TrainerPoké)
Other Links:
TRSRockin's R/B HEX List
Wikipedia's article on the hexadecimal system