Significa Inicialização Diretório (dois links diferentes da Wikipedia).
init :
init (short for initialization) is a program for Unix-based computer operating systems that spawns all other processes. It runs as a daemon and typically has PID 1. The boot loader starts the kernel and the kernel starts init. If one were to delete init without a replacement, the system would encounter a kernel panic on the next reboot.
The functionality diverged, in Unix systems such as System III and System V, from the functionality provided by the init in Research Unix and its BSD derivatives. The usage on most Linux distributions is compatible with System V, but some distributions, such as Arch, use a BSD-style and others, such as Gentoo, have their own customized version.
.d:
.d, a common pathname component suffix for a directory for a program or subsystem on POSIX and POSIX-like systems; for example "/etc/init.d" would be a directory which contains files associated with the init program.