The GNU compiler can produce two kinds of diagnostics: errors and warnings. Each kind has a different purpose:
Warnings might indicate danger points where you should check to make sure that your program really does what you intend; or the use of obsolete features; or the use of nonstandard features of GNU Fortran. Many warnings are issued only if you ask for them, with one of the -W options (for instance, -Wall requests a variety of useful warnings).
Note: Currently, the text of the line and a pointer to the column is printed in most g77 diagnostics.
See Options to Request or Suppress Warnings, for more detail on these and related command-line options.