Ah, so the Patronus is first introduced in the third book. There are these bad creatures called Dementors that suck our your happiness (and eventually your soul), and the only way to dispel (snort) them is with this a magical spell that is formed from your happy memories and takes the shape of some creature that is representative of your essence. So it’s kinda fun to try and figure it out. ^_^
…you don’t know who Tonks is either, do you?