This is a "learning" or "conditioning" term that refers to learning that two different events occur or happen together. This is really a fundamental component of conditioning since a response to a stimulus won't really be learned if the organism doesn't get the point that the stimulus and response are supposed to occur together. This doesn't have to be a conscious learning (remember, there is a big difference between classical and operant conditioning) but the association must be made for the learning to occur. For example, will a rat learn to press a lever if it never makes the association between pressing the lever and getting the reward? Or why would a dog salivate to a bell if it never makes the connection between the bell and getting food?