Bank 1 refers to the side of the engine where cylinder 1 is located. I believe this is on the passenger side for your car. I do not have my Haynes manual or service manual to confirm so double check.
For additional info about what this code means, visit:
OBD Trouble Code: P1151 Lack of HO2S-21 Switch, Sensor Indicates Lean
Simply put, they explain it a lot better than I can. They give you probable causes, what to look for, and what to check for.