A varialbe is a container for holding a value of a specific type.
If you want to sum two numbers, you might use 3 variables. 2 variables for holding two numbers that are to be added and one variable for storing the result.
In the following example, the number 4 is stored to the variable a. Number 6 is stored to the variable b. Then the values of these two variables are added and the result is stored to the variable c, so the variable c now holds value 10;
var a = 4;
var b = 6;
var c = a + b;
Remembner: Before you can use a variable you have to declare it.