The bold words are an adverbial clause , since they are a group of words that expand the information given previously. Cómo toda cláusula adverbial cumple con tres requisitos:
1- It contains a subject and a verb. The subject is me and the verb swim.
2 - Contains a subordinate conjunction , joining the two sentences with a relationship of subordination ( grammatical relationship in which one element depends on another)
3- Answer one of the questions: what, who, when and where? Why? How? Where?