As humans, we get used to things. Something that is new and incredibly exciting can become boring. This tendency to have decreased responsiveness to something is habituation (you might also hear someone say that you get habituated to something). For example, there may be a painting or picture you really like so you put it on the wall in your room. You see this picture every day, 10 times a day. Over time and repeated exposures to this picture you might start feeling like you've "seen it a million times" and it just doesn't have the same effect on you that it used to. This is habituation.