Regarding the rate of learning, the most important pairings of the cs and us are those that come

CS stands for conditioned stimulus, while US stands for unconditioned stimulus. These terms are used in the classical conditioning , which is also known as Pavlovian conditioning. It is a learning procedure in which a biologically potent stimulus is paired with a previously neutral stimulus.
Regarding the rate of learning, the most important pairings of the CS and US are those that come at the beginning of training.