In the formula in Figure 1, pi is a named constant whose value is shown. Constants in programming mean pretty much the same thing that they mean in algebraic formulas. That is, the value of a constant never changes.
The terms a and r in the formula in Figure 1 are variables. Going further, your algebra teacher may have told you that a is the dependent variable while r is an independent variable. That is to say, the value of the area is dependent on the value that you assign to the radius before you calculate the area.
Variables also mean pretty much the same thing in programming that they mean in algebraic formulas. Every variable has a name (such as a or r) and the value of the variable can change. In programming, this means that the value can change as the program executes over time.