Archiving a project consist in placing a project outside of the current list of project without deleting it, in order reduce the number of active projects in the list. Archiving a project does not delete the project, its data or its contacts.
Archive a Project
Go to All projects and click on Archive at the end of the line of the project you want to archive: 
Restore a Project
To reactivate (unarchive) a project, go to the project list and add a filter Is archived. Then, click on Unarchive at the end of the line of the project you want to reactivate: