Wikipedia
Self-hosting is the use of a computer program as part of the toolchain or operating system that produces new versions of that same program—for example, a that can compile its own source code. Self-hosting software is commonplace on personal computers and larger systems. Other programs that are typically self-hosting include kernels, assemblers, command-line interpreters and revision control software.
If a system is so new that no software has been written for it, then software is developed on another self-hosting system and placed on a storage device that the new system can read. Development continues this way until the new system can reliably host its own development. Writing new software development tools without using another host system is rare and in many cases impractical.