The verb être is used as the auxiliary verb with certain action verbs and also with reflexive verbs (doing to yourself).
The past participle must 'agree with' who did the action. So if the person was female you must add an extra -e to the end of the past participle: e.g. elle est partie - she left.
If two or more females did the action you must add -es:
If the person who did the action was one male, then the past participle does not change: e.g. il est parti - he left.
If two or more males did the action add -s: e.g. ils sont partis - they left.
If 'on' refers to more than one person, the past participle must be plural: e.g. on est allés - we went.