A variable is a letter that you can use when your doing a algebraic expression. For example, n + 17 n=11. Thats one of the ways to work with a variable. Constant is a number that cannot change.
The difference of a variable and a constant is that when a letter that represents a number that can vary, It's called a variable. Vary means that it can be different and bigger. A constant cannot change in number so thats the difference.