Whenever possible, it’s recommended to work in a common Git repository and use branching strategies to manage your work. However, if you do not have write access for the repository you want to contribute to, you can create a fork.

A fork is a personal copy of the repository and all its branches, which you create in a namespace of your choice. Make changes in your own fork and submit them through a merge request to the repository you don’t have access to.

https://docs.gitlab.com/ee/user/project/repository/forking_workflow.html

How is this different from GitHub?

Just to make sure there's no misunderstanding: When I want to contribute to a project I'm not involved in, like inkscape, I'm not allowed to create a branch in their repo, so I have to fork it, which creates a copy of the repo, and sets the original repo as a remote.

Note that git is a distributed VCS that doesn't distinguish between servers and clients. Forking and cloning are the same operation from a technical perspective, except when you git clone, the copy ends up on your local machine, and when you press the "fork" button, the copy is on a GitHub/GitLab server.