What is the difference of constant and variable?

A variable is an attribute of a physical or an abstract system which may change its value while it is under observation.

Constants are real numbers or numerical values which are significantly interesting in some way.
There are 2 kinds, physical and mathmatical. You didn't state which one so here are both.

A mathematical constant is a number, usually a real number, that arises naturally in mathematics and is significantly interesting in some way

A physical constant is a physical quantity that is generally believed to be both universal in nature and constant in time. It can be contrasted with a mathematical constant, which is a fixed numerical value but does not directly involve any physical measurement.

Hope this helps. Susu