How to say where you've been, when you arrived etc
On est parti ?
Je suis arrivée ici il y a trois ans.
Vous êtes arrivés à quelle heure ?
One way of talking about what you've done is to use part of the verb avoir with a past participle ( See chambre d'hôte) : j'ai fait, on a visité, vous avez bien dormi ? etc This is called the perfect tense and it describes completed actions.
But there are a handful of verbs that form their perfect tense with parts of être instead of avoir. These include aller, venir, arriver, partir and other verbs to do with movement.
For this group of verbs the past participle changes like an adjective according to the gender or number of the subject. These different endings only really matter if you are writing French as they don't affect pronunciation.
This tense is called the passé composé and can have several English translations. Il est allé à Paris can mean: he went, he has gone or he has been to Paris.