On: this little word has several functions and it is important to know them. If you open a grammar book, it will tell you that on is an indefinite pronoun that can be translated as “one”, “you”, “they”, “someone”.
On peut pas fumer ici.
You can’t smoke here.
Colloquially, it also means “we”. Nous, the official first-person plural pronoun, is almost never used in colloquial French. Note that the verb following on conjugates like the third person singular (il). You can forget the normal first-person conjugation (nous faisons, nous sommes → on fait, on est). (Well, don’t forget it, but you will not need it in colloquial conversations.)
On fait quoi ce soir ?
What shall we do tonight?
Note that this is only true when on/nous is the subject: as an object, nous is still used (e.g. pour nous, pour on is not correct).
Tu sais ce qu’ils nous ont dit ?
Do you know what they told us?
The reflexive pronoun, however, is se.
On se connaît ?
Do we know each other?
In this sentence, on nous connaît would mean “someone/they know us”.
Listen to the examples: