It is not completely clear to me that what "interface" means in Fortran is what you are asking about, but the simple answer is that Fortran allows you to put procedures in modules, which then requires that the caller of the procedures add a USE statement for the module. This provides an "explicit interface" (Fortran term) that supplies details about the procedure's arguments to the caller. Grouping related procedures and other declarations in modules is the recommended practice in Fortran - it makes the program easier to maintain and allows the compiler to better check for errors. Explicit interfaces are also required to make use of newer language features such as assumed-shape arrays.
