Recently I navigated to the SO chat, and I noticed some of chat rooms have a Locked icon at Top Left side corner.
What does this locked icon mean?
If you hover over the padlock, it says:
Gallery: anyone may enter, but only approved users can talk
Essentially, a semi private chat room, a lot of the times for specific topics between 2 or more members (I would imagine an attempt to keep it on topic as much as possible). If you click on the room, there is a button to request access.