The video gallery is where you'll see everyone's cameras in the room.
When there's too many people to show at once, you can click on the arrows on either side of the gallery to page through the videos. If the person speaking isn't already in your video gallery, we'll pop their video out to the side so you can see them.
The three dots menu allows you to change video gallery size, hide self view and adjust which participant videos appear including all participants, active speaker, or a specific participant.
There are four video gallery options. The first three (left, top, and right) will pin the videos to each side of the room. This leaves plenty of space on the screen to focus on the room.
When the main focus is the conversation, you can use the last video gallery option to make the videos full screen.
Additionally, when you click on someone's video a menu will appear. Here you can follow the participant, request they mute or unmute themselves, and request they enable or disable their video. If they are a Guest in the room, you can also disable their shared control of the room from this menu.
You can mute your microphone and camera by clicking on the corresponding buttons centered in the room toolbar. 
Clicking on the down arrow just to the right of each button opens their settings where you can change inputs, or apply a video background.