A "screenshot" is a picture of whatever is on your computer screen at the time you do it. You hold down the "shift" key while simultaneously pressing the "PrtScrn" key and it will record a image of whatever is on your screen to the clipboard. Then you can paste that into any graphic-capable program- like Word, Wordpad, Paint, or any photo editing program. It's something that every Windows computer can do and doesn't require any special program.
Below is a screenshot (in reduced size) of my computer desktop-