The getKey() command returns the key code of the last keypress. If no key was pressed since the program, function, or expression started running, or since the last getKey() command, getKey() returns 0. It's important to note that once getKey() is used, the keypress is forgotten — even if it's used in the same line! So most of the time you want to store the result of getkey to a variable to use it.
The keypresses that getKey() deals with factor in modifier keys, such as 2nd or alpha. Because of this, it will not respond to the modifier keys pressed by themselves.
This example code using getKey() is commonly used in programs that wait for the user to press a key:
:0→key :While key=0 : getKey()→key :EndWhile
Although the key codes are given in a table on this website, and are listed in your manual, it may be more convenient to write a short function to return key codes for you:
:keycode() :Func :Local k :0→k :While k=0 : getKey()→k :EndWhile :k :EndFunc
If you run the function keycode(), it will wait for you to press a key. When you press it, it will return the key code. This function may also be a convenient subroutine in a program that requires waiting for a key in several different places.