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 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?