This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb.

The kanji 外 means outside, but this word means to be disconnected, to come off, or to be off. It's used when something separates from or comes off of something else. It's intransitive, meaning that it happens on its own. You don't do it, but it just disconnects itself.

You already learned 外す, which is used when YOU disconnect something. Like many transitive verbs, that one ends in す. 外れる, on the other hand, means that something disconnects all on its own. To help you remember the difference, imagine yourself walking up a flight of stairs and the rail (れる) coming off in your hand. Oops! You didn't do that, it happened all by itself.