What is a LAMDA sensor and where is it in a 1jzgte?

quoted:
for closed loop control of the air-fuel ratio, and fault sensing of the catalytic converters, the exhaust system is equipped with two lambda sensors. one mounted ahead of the first catalytic converter, the other o2 sensor is just after the 2nd cat. only the 1st o2 sensor provides feedback to the ECU for closed loop fuel trim. apparently the 2nd sensor is used for fault detection of the cats.