The term byte represents an ordered collection of bits, with each bit representing a binary value of either 0 or 1. In most modern hardware, the defined size of a byte is eight bits, hence usually there are 2^8 (or 256) distinct values in one byte.
In the Pokémon games, the smallest groups of identifiers can only be read by the console in single 8-bit bytes. This explains for example, the reason why there are 105 glitch Pokémon in Generation I, because 2^8 is 256, and 256-151=105.