A fork is a copy of an original repository that you can put in another namespace where you can experiment and apply changes that you can later decide if publishing or not, without affecting your original project.
It takes just a few steps to fork a project in GitLab.
Go to a project's dashboard under the Project tab and click on the Fork button.
You will be asked where to fork the repository. Click on the user or group to where you'd like to add the forked project.
After a few moments, depending on the repository's size, the forking will complete.