When you are in a zoom video meeting, "active speaker" will automatically switch any user that is speaking to that main video screen. You must have 3 or more participants in your meeting for "Active speaker" to work.
When you click on ("spotlight") any users video window, it will turn off active speaker, and remain on that users video.(indicated by green highlight around the window)
To turn active speaker back on anytime during the meeting, you must click on the "Switch to active speaker" icon.
Note: "Active speaker" is always on by default when starting or joining a zoom.us video meeting.