To Rot
intransitive verb, godan verb
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means rot so the verb vocab version is to rot.
Remember the word 臭い, which means "stinky"? When something rots, it becomes stinky, and all くさい.
牛乳は腐りやすいんだから冷蔵庫の外に出しっ放しにしないでね。
Don’t leave milk out of the fridge because it goes bad easily.