Variable is kind of memory space, which is used to store values that are changing from time to time. For example if x is an integer variable you can say:
x=25+10;
What really happens in the computer is, the ram reserves a memory space for integer value 35 and it puts the memory location(address) of the value in the variable x. Then again you can assign a new similar type value to x, hence the name, x's value varies time to time .
x+=20;
But if you declare a value as constant, it cannot be changed at different statements. For Eg: in maths, Pi's value remains same at all times because Pi is a constant. Variables and constants have totally different uses in programming.
Hope this helps.