Dishabituation is when we respond to an old stimulus as if it were new again. When we repeatedly see or experience a stimulus, our response to it grows weaker. For example, you play peek-a-boo with a baby by covering your face with a blanket. The first time you pull down the blanket to reveal your face, you can expect to get lots of squeals and laughter. But if you keep doing this, the baby's reaction will eventually decrease, until you can barely even coax a giggle or a smile out of him.
However, if one time you pull down the blanket and instead of your face, the baby sees a hand puppet, he will likely squeal in delight again, just like he did when you first initiated the game. Now when you put up the blanket and then pull it down again to once more reveal your own face, you can expect the baby to have a renewed interest in you.
When we repeatedly experience a stimulus, we eventually get used to it, and stop responding the same way we did when we first encountered it. We become habituated to it and stop paying attention until we are given a new stimulus. And then when we are once again given the original stimulus, we respond to it with a renewed interest. Our previous habituation to it has been reversed. That is called dishabituation.