.d suffix here means directory. Of course, this would be unnecessary as Unix doesn't require a suffix to denote a file type but in that specific case, something was necessary to disambiguate the commands (
/etc/rc1 and so on) and the directories they use (
This convention was introduced at least with Unix System V but possibly earlier. The
init command used to be located in
/etc but is generally now in
/sbin on modern System V OSes.
Note that this convention has been adopted by many applications moving from a single file configuration file to multiple configuration files located in a single directory, eg:
Here again, the goal is to avoid name clashing, not between the executable and the configuration file but between the former monolithic configuration file and the directory containing them.