An example of a keyword is the var command. Var tells Flash that the string of characters after the keyword should be defined as a new variable.
In the image above, I've used the var command to define a variable called myvariable; I've used another keyword, int, to set the variable type as an integer. Other examples of keywords include gotoAndPlay, gotoAndStop, stop(), play(), and function.
Keywords commands are already defined within ActionScript 3.0, and can't be used for any other purpose; you can't assign a keyword as a variable name, for example, because then it will stop meaning what it should and will confuse Flash. Just as you can't decide that gracias means horse, you can't name a variable gotoAndPlay; the gotoAndPlay command already has its own meaning in Flash's language, and can't take on a new one.