You can read more about Snapper here.
Snapper is a tweak that lets you copy any part of the screen and have it stay on top of the screen even when you change applications. As an example, this is great for when you have taken a photo of a WIFI password and need to enter it in the settings app.
You can see Snapper in action in this video.
It also lets you save the "snap" to the camera roll, or copy it to the clipboard.