Sometimes I stumble across a really simple concept that I feel like I should have known, and perhaps even at one point did know. "Blocking" is just one of those concepts, which I found in David Shanks' Annual Review of Psychology paper Learning: From association to cognition.
The basic idea is pretty simple. In classical conditioning, when a neutral stimulus is presented alongside an unconditioned stimulus that generates an unconditioned response, that neutral stimulus normally becomes a conditioned stimulus that causes a conditioned response. The classic example is, quite obviously, Pavlov's dogs; the smell of food (US) caused salivation (UR). When a bell was rung (NS) at the same time as the dogs' food was served, the dogs eventually began to salivate when they heard the bell - a conditioned response. The dogs had learnt to associate the bell with being fed.
That's all well and good. If, however, a second neutral/conditioned stimulus is introduced after an association has been made between CS1 and the US, and presented consistently with CS1, then little to no association will be made between CS2 and the US. To demonstrate with a thought experiment; if I make an association between, say, eating peanuts and having an allergic reaction, then experiencing an allergic reaction after eating peanuts and drinking beer won't make me associate beer with an allergic reaction. The association between CS2 (beer) and US (allergic reaction) has been blocked.
It's simple enough, but I found the effect interesting due to the implications it raises as to how the learning process actually works in humans and other organisms.