Acquisition refers to the initial stage of the learning or conditioning process. In this stage, some response is being associated with some stimulus to the point where we can say the organism (person, animal, etc.) has “acquired” the response. During this stage, the response is strengthened (reinforced) so that it is truly “learned”.
For example, if you are trying to train a rat to press a lever in response to you ringing a bell (i.e., trying to condition the rat to press the lever when and only when you ring the bell), then once the rat presses the lever in only response to the bell, you can say the response is “acquired”. You would then continue to gradually reinforce the lever pressing in response to the bell to make sure the response is strengthened.