"Leaving the project" refers to the action of no longer being a member of the project.
There are two ways to leave a project:
1) On the catalog home page, click on the cogwheel at the top left of the project thumbnail.
2) In a project, by clicking on the 3 small dots. They can be found to the right of the "Express yourself" box.
A message appears to confirm that the project has been exited: