Private teams can only be joined if the team owner adds someone to them. When creating a team, choose to make the team discoverable so that people can search for it in the teams gallery, or turn discoverability off so that people can only find the team if shared by you or a team member.
Public teams are visible to everyone from the teams gallery and you can join them without getting approval from the team owner.
Change a team's privacy status
To change the privacy settings for your team from public to private (or vice versa), go to the team name and select More options > Edit team toward the bottom of the menu. Under Privacy, select either Publicor Private.
You can now choose to make private teams discoverable when people search for teams to join. People will still require permission from a team owner to join a private team, this only allows the team to turn up in search results for your org.
Note: To change a team's discoverability, right-click on the team name, then select Manage team > Settings > Team discoverability.