Archiving lets you keep your old completed projects in the application. Projects can be restored if you ever need to continue working on a previously archived project.
How to archive a project?
Navigate to your project list, and select the vertical ellipsis to the right of the line item and choose Archive.
To access your archived projects, change the Status filter at the top of your screen to Archived. There you will see a list of all archived projects for your org.
To restore/unarchive a project, refer to the article, How to unarchive/restore a project?