The spell actually means ‘I await for a protector” in Latin. In the Harry Potter itself, ‘Expecto Patronum” is used to repel Dementors. So, in other words, the spell is used to repel all the bad aura that wants to steal happiness from me. The unique thing about it is, if you want to cast the spell, you have to think of things that make you happy. The happier your memory is, the stronger your patronus will be.