Learn French: Elles sont enfin rentrées.

Because rentrer (to go home/return) is a verb of motion. In the past tense (passé composé), a specific group of motion and state-change verbs uses être (to be) as the helper verb instead of the usual avoir (to have).

Why is there an -es at the end of rentrées?

When a past tense verb uses être as its helper, the main verb (the past participle) must agree with the subject. Since Elles (they) is feminine and plural, we add an -e for feminine and an -s for plural to the basic word rentré.

Why is the word enfin (finally) placed in the middle of the verb?

In French, short and common adverbs like enfin are usually placed right between the helper verb (sont) and the past participle (rentrées).

How does rentrées mean "went home"? I thought it meant "re-entered".

While rentrer literally means "to re-enter" or "to return", it is the standard, everyday verb French speakers use to say "to go home" or "to come home".