You can define a variable simply by using the command var followed by the variable name and a semicolon; this instructs Flash to establish a variable of a specific name. In the image above, I create a variable called myvariable, then define it as a particular type (integer). On the next line, I make myvariable equal to the number 42.
Another example of a variable might be a user's name. If you create a Flash script that collects the user's name and then displays it later, you might assign a variable called username to the input field where the user types in their name. The value of username would change every time a new name is typed in. You can also assign variables to things like frame numbers, object / symbol sizes, and numerous other things - all of which might change during the course of the Flash movie, thus changing the value of the variable.
You can think of variables in a fashion similar to the classic x in algebra, which is usually most people's first encounter with the concept of variables. You probably groaned the first time you saw x = y + 1, but the concept is the same when applied to variables in ActionScripting. The variable x is defined to have a specific value, which can be constant or can change depending on the value of another variable, y.