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.  

In this video I go over the general process of learning how to one look a 2x2 solve. 

In this video I go over a more advanced one looking technique. One looking the AUF of your 2x2 solve.