A screenshot, screen capture, or screen dump is an picture of a computer screen and the information it is displaying.
How to make a screenshot: Press PrtScrn/SysRq.
Nothing will appear to have happened, but your computer just took a snapshot of its screen and stored that picture on its clipboard.
You can then paste this image into an email or document by pressing Ctrl+V or right-clicking and choosing Paste.
Further instructions for Windows, Mac and more can be found at take-a-screenshot.org