On (listen) is the indefinite pronoun and literally means "one." It's often equivalent to the English passive voice.
On ne devrait pas poser cette question.
One shouldn't ask that question.
On demande : caissier.
On ne dit pas ça.
That isn't said.
Ici on parle français.
French is spoken here.
In addition, on is an informal replacement for "we," "you," "they," "someone," or "people in general."
On va sortir ce soir.
We're going out tonight.
Alors les enfants, que veut-on faire ?
OK kids, what do you want to do?
On dit que ce resto est bon.
They say that this restaurant is good.
On a trouvé mon portefeuille.
Someone found my wallet.
On est fou !
People are crazy!
On ne sait jamais
You never know
Agreement with on
There are two related debates about whether agreement is required with the subject implied by on:
Adjectives: In On est content (We/They are / Someone is happy), should the adjective agree?
feminine: On est contente.
plural: On est contents.
feminine plural: On est contentes.
Être verbs: In On est tombé (We/They/Someone fell), should the past participle agree?
feminine: On est tombée.
plural: On est tombés.
feminine plural: On est tombées.
There is no real consensus, so here's my opinion: On is a neuter singular pronoun, so there shouldn't be agreement, but it's pretty much up to you - or your French teacher. ;-)
Quiz on on
Related lesson: On vs l'on