Private channels was introduced around new-year 2020, and since then this is a new option when you want to difference who can access what in Teams.
Before Private Channels you need to create a new team in Teams if you have needs for different access rights, but now there is a new way. You can have one Team with different user access rights. E.g. all members do not have the same access rights!
When you create a new PRIVATE channel in Teams, only you have access to it. Even if you have other members already in the team. The private channel do not inheritage the security from the team. To let other people acces the private channel, you must add them as members or owners to the private channel. But you can only add people to the private channel from already existing people in the team.
When the private channel is created you can work with it almost excatly as you do with a standard channel. Some features cant be used, one of them is Planner - you cannot have a Planner plan for only people in the private channel. Over time we can expect that Microsoft will solve those restrictions the private channels do have today, but for now I have not found any workarounds on that.
Conclution
With Private Channels in Teams you can restrict who can access content to only a subset of people that otherwise have access to the team. This option gives you an alternative to create many teams, you can select to have fewer teams and private channels in them instead.
