A screenshot is what it says, a shot or picture of what is on your screen. To practise this, press the Print Screen (sometimes abbreviated to Prt Scr or similar) key on your keyboard then open up a graphics application on your PC such as Microsoft Paint, then hold down the CTRL key and press the V key and a picture of what was on your screen when you pressed the Print Screen key will appear in Microsoft paint.
To upload that picture to the forum is another story but knowing how to do it all is a useful string to your bow.