A variable is a value that is not constant. This value is changed conditionally. For example weather is a variable, it is not same always.
Now in SWiSHScript, for example "X" is a variable and its value is assigned x=10; in the beginning of Movie, but we want to change its
value at runtime (means you can change value while your SWF file is playing), so we will simply assign a new value to "X" as x=20; at any frame.