Guys, seriously? A/F and oxygen sensors are entry level. Anyway, Bank1 refers to the side of the engine that contains cylinder1(in this case, left side, towards front of vehicle). Bank2 refers to the side of the engine that contains cylinder2(in this case, right side, towards rear of vehicle). I got you a section of Lexus repair manual for a 2001 ES300. Otherwise, the code is the same for most models/years. Don't over analyze it, though. This code means you need the sensor.
Thanks for the reply. So this is not bank 2, sensor 1:
Sorry dude, I got that backwards. Bank2 is towards the front, Bank1 is towards the rear.