Spanish verbs can be broken into different categories, here you have the "reflexive" Those that require the use of a reflexive pronoun (me, te, se, nos, os, se) A reflexive verb is required when the speaker is doing the action to him/herself, to each other, the subject of one, or to express the passive voice :)

le dolía la cabeza >> le = indirect object pronoun 3rd person singular; this verb is a bit tricky, 'la cabeza' is the subject of the verb and the person with the headache ('le') is the indirect object, so you need 'le' to know who has a headache; 'doler' follows the same pattern as the verb 'gustar' in Spanish se quitó la chaqueta >> se = reflexive pronoun 3rd person singular; you use 'se' here to indicate that someone is both the subject and indirect object of the verb, they do the action to themselves

Le dolía la cabeza In this case LE is an article used to indicate that it is just one person, if there was two or more it should be Les dolía la cabeza. Here are all the examples: (yo) (I) Me dolía la cabeza (tu) (you) Te dolía la cabeza (el) (he) Le dolía la cabeza (ella) ( she) Le dolía la cabeza (nosotros) (we) Nos dolía la cabeza (ustedes) (you/ plural) Les dolía la cabeza (ellos) (they) Les dolía la cabeza Se quito la chaqueta means an action he did to himself, without the "se" it could mean he took the jacket away from somewhere else. Examples Juan se quito la chaqueta. Juan quito la chaqueta de el ropero in spanish if you only say Juan quito la chaqueta.... it is incomplete cause you need to point out from where.

gracias jader!

Some examples: It's related = Se relaciona I was told = Me dijeron/contaron It will be carried out = Se llevara a cabo In spanish is too coomon using this structure and instead of english, we strike or omit the subject.. greetings!! Jader from Colombia