Pop one string, search for a variable of that name, and push its value on the stack. This action first checks for local variables in the current function. If there isn't such a variable, or the execution is not in a function, then the corresponding global variable is read.
The variable name can include sprite names separated by slashes and finished by a colon as in. Only global variables are accessible in this way.
Example:
/Sprite1/Sprite2:MyVar
In this example, the variable named MyVar is queried from the sprite named Sprite2 which resides in Sprite1.
In a browser you can add variables at the end of the movie URL (as defined in the W3C docs) and these will automatically be accessible via this Get Variable action.
Example:
my_movie?language=jp
Defines the variable language and sets it to "jp".
Since Flash V5.x, there are internal variables available to you and these can be read with the Get Variable action.