It's the third-person singular form of the verb "doler", which means "to hurt" or "to ache".
Here are some examples:
* Me duele la cabeza. (My head hurts.)
* Duele la garganta. (The throat hurts.)
* Duele la espalda. (The back aches.)
Note:
* "Duele" can also be used to express emotional pain, like sadness or heartbreak.
* The verb "doler" is used with the preposition "a" when referring to a person. For example: "Me duele a ti" (I hurt for you).