ELI5: Explain Like I'm 5

MissingNo.

Missingno. is a glitch in the old Pokémon games that happens when there is a mistake in the game programming. It stands for "missing number" because it appears when the game tries to load a Pokémon that doesn't have a number assigned to it.

When you encounter Missingno., it looks like a jumbled mess of pixels that kind of looks like a bird or a block. Sometimes it can look like other Pokémon, but it's not really a Pokémon at all. It's more like a mistake or a glitch.

If you catch Missingno. or defeat it in battle, strange things can happen in the game. For example, your items might get duplicated, or you might get a lot of rare candies.

While Missingno. is a cool and interesting part of Pokémon history, it's important to remember that it's not a real Pokémon and was never meant to be in the game. Trying to catch it or use it too much can also cause glitches and problems in your game, so it's best to just let it be a fun little oddity and not try to mess with it too much.