What is "One Looking"?
One Looking is where you predict your solve from start to finish. So, in a 2x2 solve, instead of just predicting your layer/face you will predict everything after that also. By one looking your solve your solves should have almost zero pauses in them.
When should I learn to One Look?
I would say learn it as soon as possible! I waited until I averaged around 2.6 to really learn how to one look and in my opinion that was a big mistake.