It means in French, let's leave. Se casser in French really means to break. But in slang, se casser means to leave. On is a familiar way of saying, We. Thus: Let us (we) leave. The se is a grammatical word and this is not a French lesson so let us leave it at that.
Cette boum est morte, on se casse!
This party is dead, let's leave!
This party is dead, let's get the hell out of here.
Voilà la police, on se casse!
There is the police, let's get the hell out of here!
