A variable is a symbol that can be assigned an arbitrary value. A "symbol" is a series of alphabetic and numeric characters, separated by whitespace (a space, a line-break, or the end of the file). When a variable is "assigned" a value, the variable name (the symbol) is understood to be a substitute for the assigned value.

Consider a traffic light, which has three possible symbols: green, yellow, and red. When you are driving, and you encounter a traffic light, you might see that its red symbol is activated (the red light is illuminated). What you see is a red light, but you understand that it means you should stop your car. Red lights in general do not make you stop - it is specifically red traffic lights, because we know that it is a symbol meaning to stop.

SuperCollider's variables work in the same way: you tell the interpreter that you want to use a symbol, like cheese. Then you assign cheese a value, like 5. After that point, whenever you use cheese, the interpreter will automatically know that what you really mean is 5.

Run the following two programs. They should result in the same output.

(
  5 + 5;
)
(
  var x;
  x = 5;
  x + x;
)

In the first example, the program calculates the value of 5 + 5, which is 10, and returns that to the interpreter, which prints it out. In the second example, the program tells the interpreter that it wants to use a variable called x then it assigns cheese the value 5. Finally, the program calculates cheese + cheese, which it understands as meaning 5 + 5, and returns 10 to the interpreter, which prints it out.

This trivial use of a variable does nothing but complicate the process of adding 5 to itself. Soon you will see that variables can greatly simplify your programs.