The Chatter Free License is for users who don't have Salesforce licenses but must have access to Chatter. These users can access standard Chatter items such as people, profiles, groups, and files but they can't access any Salesforce objects or data. Chatter Free users can also be Chatter moderators.
Chatter Free users don't see tabs like other Salesforce users. Chatter Free users access feeds, people, groups and files using the App Launcher in Lightening Experience. In Salesforce Classic, users access these features from links in the page sidebar.
Salesforce administrators can upgrade a Chatter Free License to a standard Salesforce or Lightening Platform Starter license at any time. You can't convert a standard Salesforce, Lightening Platform Starter, or Chatter Only license to a Chatter Free license.