The clipboard is where your computer stores things temporarily. As in when you turn off your machine or do something else that writes to the clipboard what you had before is lost.
You know how you can copy and paste lines of text or even cut and paste them? Well the place that they are stored when they are between the cut and pasted steps of the process is the clipboard.
Same deal goes with screen shots. If you hit print screen don't go looking around for your picture or expect anything else to happen. You have to go to an image editor and paste the picture there yourself.
A more generic way to think of the concept of the clipboard is as a tiny amount RAM.