What is git hosting?
Git hosting is where the repositories live. The pipeline starts from there.
What it is
Git keeps the history of your code on your machine. Git hosting is the server where that history lives for everyone. Locally you have a copy. A push brings commits there. A pull fetches what others have pushed.
Local is a copy. The host is the place where it is settled what is on main, and what the pipeline starts from.
What it is made of
Without that place there is no shared history. Everyone works in their own folder, and merging becomes a file someone sends by mail. With hosting there are branches, merge requests, and one place where it is clear what is on main.
What goes wrong
The pipeline starts from there too. A push to main is the signal: this state should be checked, built, and shipped. If the code only lives locally, nothing happens after you save, except on your disk.
What you otherwise look after yourself
Access belongs to it. Not every account may write to every repository. Deploy keys and tokens are access for the pipeline, not for people. A token in chat or in the repository is an access you cannot take back once it is in the history.
How it shows up on MainPath
On MainPath every building block has a repository in GitLab. How saving and publishing fit together is described under Save and publish.