HO2S= Heated Oxygen Sensor, sensor used to measure the amount of oxygen in the exhaust, which is used to adjust the air/fuel mixture in the engine.
B2= Bank 2, this is a location identifier, the engine has 2 banks, 1 and 2. Bank 1 refers to the bank nearest cylinder #1.
S1= Sensor 1, all engines have an oxygen sensor before and after the catalytic converter, S1 refers to the sensor before the CAT, S2 is after the Cat. Conv.
The sensor uses a heater element, similar to a cigarette lighter. The O2 sensor needs to be hot for it to work, and the heater ensures that the temperature stays above that operation range.
The code is often set when the heater is bad, and the sensor would then need to be replaced.