The C interoperability features have several parts. The module ISO_C_Binding is just some declarations of constants, types and functions that are useful in Fortran code for interoperating with C. The BIND(C) “language binding spec” is another important part.
Fortran 2018 greatly extended C interoperability, by allowing you to pass a wider variety of things between Fortran and C (in both directions). This is enabled by a C include file called ISO_Fortran_binding.h, which declares macros and functions for manipulating what Fortran calls “C descriptors”. With these, C code can accept and pass allocatable and pointer variables, arrays with bounds, character strings with length, and more.