The concept of a variable needs to be introduced first. A variable is a storage location with a name. You can store a value in a variable, and you can later examine and use that value later. Fortran allows the use variables. But each variable that is going to be used in a Fortran program should be declared. In other words, before you use a variable in a Fortran program, you should tell the compiler something about that variable. You should tell the compiler two things about every variable: the name of the variable and the type of the variable. The statement implicit none forces the compiler to look for a definition of every variable.