How does the finger scanner in iPhone 5s work?

I just got the 5s today and it had me scan my fingerprint but I thought it was to like unlock your phone? It only lets me have a number code or none at all? How do I set it to where I have to scan my finger to unlock it? Isn't that what it is for? I'm so confused.

Ok Ill make it short go to Setting>General>(Scroll Down)Touch Id & Passcode>Enter passcode and click touch id and there you go

You can set it to fingerprint, code, or both. When recording fingerprint do the thumb 4 times, don't do different fingers like some are doing, its far more accurate just to do the same thumb 4 times etc.