Explanation
Private Channels in Microsoft Teams create focused spaces for collaboration within your teams. Only the users on the team who are Owners or Members of the Private Channel can access it. Anyone, including Guests, can be added as a Member of a Private Channel as long as they are already members of the team.
You might want to use a Private Channel if you want to limit collaboration to those who have a need to know or if you are going to facilitate communication between a group of people assigned to a specific project without having to create an additional team to manage.
What Is Different?
Currently, Private Channels support connectors and tabs (except Planner and Forms). Each team can have a maximum of 30 Private Channels, and each Private Channel can have a maximum of 250 members. Scheduling a meeting in a Private Channel is not supported. You can schedule an appointment in Outlook outside the channel and then invite the people you want to schedule the meeting with.
You can see that Private Channels are marked with a locked button beside the channel name in the below image.
Note: If users select a Private Channel, it can not be changed to a Public Channel. Once created, a Private Channel will remain private. This also applies to Public Channels as well.