In Le Passé Composé, most verbs use avoir as the auxiliary verb but a fixed number of verbs use être instead.
They are almost all verbs to do with movement (See Conjugate coming and going verbs (+ être) in Le Passé Composé (conversational past)), but also those to do with states of being (birth, death, becoming, staying).
Have a look at the following examples:
The pattern for these verbs in Le Passé Composé:
être (le Présent) + past participle of the verb
Because they're using être as auxiliary verb, their past participles have to agree with the subject of the verb.
List of the "being and state" verbs and their past participles