A fork of Rural Dictionary
It's an european clothing retailer with shops placed all around the world. They sell cheap and nice clothes, hence the popularity among teenage females. It originated in Sweden.
Did you know that H&M is more correctly pronounced: "Hennes och Mauritz"? That's 'cause "och" means "and" in the swedish language. A common mistake is that people say "und" instead, which isn't right as it's a german word, not swedish. Many non-european people don't have a clue how to pronounce it that way, so just feel free to say "Hennes AND Mauritz", or just "H&M" so you don't end up making yourself look stupid.