A small group of verbs — most of them about moving from one place to another, or a change of state — builds the passé composé with être instead of avoir:

Verb	Paired with
aller → allé (go)	venir → venu (come)
arriver → arrivé (arrive)	partir → parti (leave)
entrer → entré (go in)	sortir → sorti (go out)
monter → monté (go up)	descendre → descendu (go down)
naître → né (be born)	mourir → mort (die)
rester → resté (stay)	tomber → tombé (fall)

Add rentrer (go home), revenir (come back), retourner (go back) and devenir (become). Picture a house: you arrive, go in, go up the stairs, come down, go out and leave — or you simply stay.

Je suis allé au marché. — I went to the market. Elle est arrivée hier. — She arrived yesterday.