Missingno is a glitch "pokemon" that existed in the classic games. It could be use to level pokemon beyond 100, get good items, etc. But it risked ruining your game.
Missingno is this wierd thing you can find in Red and blue Version. It does weird stuff to your pokemon like turn them into other things and make them learn wierd moves. It's considered a glitch, and nintendo fixed it for FR/LG.