A screen shot is a useful tool to help someone see what is happening on your screen. You end up saving it as a picture file that can then be emailed to someone.
To take a screenshot on a Windows 10 Machine:
On your keyboard there is a button that says "Print Screen" or "PrtScr". Find it and press it at the same time as the Windows key.
This will make the screen darken momentarily and save a screenshot in your "Pictures > Screenshots" folder. Keep in mind this will capture all monitors at once.
If you need to copy only a certain area of the screen, instead press Windows Key + Shift + S all at the same time.