Join before host allows attendees to join the meeting before the host joins or when the host cannot attend the meeting.
When you are scheduling a meeting as the host, you have an option to enable "join before host" in the Schedule a Meeting settings.
If you select "join before host," then the participants can join the meeting before the host joins or without the host.
If you do not select "join before host," the participants will see a pop up dialog that says "The meeting is waiting for the host to join." If you are the host, there is a login button to login and start the meeting as the host.
Note: If the host does not join the meeting, there are no host controls such as recording, mute/unmute all, lock meeting or expel attendees.