If you start typing the name of the command or system variable on the command line, the autocomplete feature displays your choices. For example, if you enter dim, all commands and system variables that begin with those letters are listed as shown.
Keep typing to narrow your choices, or use the arrow keys to move up or down the list. Press F1 to display the Help topic for the command or system variable that you choose. You can also hover over a tool on a toolbar or the ribbon and press F1.