Stock prices all over the world fell.
My temperature dropped to 37℃.
Unfasten your belt so that your pants will hang down.
This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb.
This is an intransitive verb. That means it does not take a direct object. The kanji means below so this means something gets lower, by falling or dropping. For example: Prices get lower or fall. Squirrels come down from trees. See? No direct object here. That's why this verb means to get lower or to fall.
Since this word has okurigana (hiragana attached to the kanji) you know that it's probably going to be the kun'yomi reading. Let's use a mnemonic to remember the reading of the kanji:
You know it means to fall so instead of using a saw (さ) to cut down the tree, you decide to let it come to you. It has to come down all by itself. Picture yourself placing a saw at the foot of a tree then sitting back and waiting for it to drop itself at your knees. (This could actually work, don't you judge my methods!)