Okay kiddo, so you know how some words have little dots on top of certain letters? Like the word "über"? Well, in music and specifically in heavy metal, they sometimes put those dots on top of the letter "u" in a band or song name, like "Motörhead" or "Mötley Crüe".
It's called a metal umlaut, and it's kind of like a cool way to make the band or song name look and sound more tough and aggressive. It's like wearing a cool leather jacket or having spiky hair - it just makes you look and feel more like a badass.
So basically, a metal umlaut is just a fancy little dot that gets added to a word to make it more cool and metal. And who doesn't want to be cool and metal, right?