The Default Team is where contacts are listed if there is no assigned team. By default, all contacts are automatically listed in the Default Team unless otherwise configured. If a contact is on the Default Team, the Team field will be left blank.
If you are looking to run a Query or a Report based off contacts in the Default Team, you would need to set the filter for Team as Blank (shown below). This will show all contacts that don't have an assigned Team, thus showing only the contacts in the Default Team.
Selecting Default Team from a Query
Selecting Default Team from a Report