could anyone tell me what a lambda and lambda sensor is?

i am a young wrx driver who is in serius need of knowladge, pls help

 

My understanding is that it is an oxygen sensor in the exhaust. My RS4 has one before the cat (to check engine is running properly) and one after the cat to check if the cat is doing its job properly. Any probs and it flags it up.

It uses the data from the first one to adjust mixture etc which is the most important thing.

D