My 2000 Maxima is wasting way too much gas

I filled up my tank and only got about $100
on that fill up i went to AutoZone and
checked my check engine light and it came
back  with 2 probable causes:
1. Open or short circuit condition and failed
H02s11 (bank 1 senser 1)
2. Fuel sytem lean bank 1.
What does this even mean and is this the
correct reason my car is wasting so much
gas?  Thank You for your response in
advance