Next on the Zoom Meeting Controls toolbar is the Video on / off button.

A screenshot from the Zoom Meeting Controls toolbar. A video camera icon is overlaid by a red diagonal line.

Clicking on the icon toggles your video on and off. 

A screenshot from the Zoom Meeting Controls toolbar of a video camera icon.
If the camera icon has a red line through it, your video is currently turned off and other participants cannot see you. 

If the camera icon does not have a red line through it, your video is currently turned on and other participants can see you.

Clicking the arrow to the right of the video icon opens a video settings menu.

A screenshot of a Zoom meeting video menu.