Commands are executed when you select them. The most obvious way to select something on the screen is to click on it - which means point the mouse cursor at it and push/release the left mouse button.
You can also make menu selections with keyboard accelerators.
These specially defined sequences of keystrokes have the same effect as mouse clicks, but are often quicker because you just press keys without moving your hands from the keyboard.
For example, to select and execute the File Load command, press the Alt key, then the f key, then release both keys. This selects and displays the File menu. Now press the l key to select and execute File Load.
There are several ways to execute commands, and each has its advantages. Often one is quicker, more natural, or more convenient. Sometime a command feature is available only when the command is is executed in a particular way - you can't enter arguments with the mouse, for instance.
You can execute commands in the following ways:
Click a command category in main menu, then an entry in its pulldown menu (RunStart).
Enter a dot-command in the upper text area (.rs).
Type an accelerator key sequence (Alt-r, s).
Type a hot key synonym key (F1).
Click a hot-button (Start).
You can enter a dot command in the upper text area to execute any pulldown menu command. Just type a dot plus the two letter command abbreviation in the upper text area.
If the text cursor is in the main text area, you can press the Escape key first to move it to the upper text area. You don't have to enter the optional argument after the dot command - a prompt window will appear and ask for the information. But you'll find it's much faster to enter argument too, since you can do everything with keystrokes.