An umlaut is a special thing we can put on top of some letters in some languages, like German. It looks like two little dots on top of the letter. This special thing can change how we say the letter.
For example, if we have the letter "a" and we add an umlaut, it becomes "ä". This makes the letter sound different, like when you say "cat" with a meowing sound.
We use an umlaut to show that we need to say a letter differently than usual. It's like putting on a special hat or costume to become a different character!