Projects are tags that you can tag your records with. Once a record is tagged with a project only users that have been granted access to that project will be able to see and edit the records tagged with that project. 

You can learn more about projects, as well as creating, editing permission to projects, and disabling projects for apps on our projects help article. 

When a record is tagged with a project and projects are enabled on an app you will see these details within the record's metadata section.

When are record is being edited/created on the web app, you will be able to edit what project the record is tagged with in this metadata section. On the mobile apps, the project field will be displayed at the top of the record just below the status field.