Learn Russian: Что ты привезла из отпуска?

It uses the arrival prefix при- added to the verb везти (to transport or carry by vehicle). Together, привезти means "to bring (by vehicle)". Since you usually travel from a vacation by transport, bringing things back uses this verb instead of the walking equivalent (принести).

Why does привезла end in -а?

This is the past tense form for a female subject. The speaker is talking to a woman. If ты (you) was a man, the question would be Что ты привёз?

Why is it из отпуска instead of just отпуск?

The preposition из (from / out of) always requires the Genitive case. To make the masculine noun отпуск (vacation) Genitive, we add -а to the end.

Is привезла a perfective or imperfective verb here?

It is perfective. Adding a prefix like при- to a one-directional verb of motion (везти) creates a perfective verb. This makes sense here because the action is completely finished—the vacation is over, and the items have successfully arrived!