Hi, 

A few basic prerequisites to consider:
1. Use a wired connection and/or 1.5 Mbps uplink/downlink for HD quality

2. Have the video clip stored locally if possible
3. Zoom recommends sharing videos with1080p resolution or below.

Starting a new screen share and optimizing for full-screen video

1. Click Share Screen 
    
    located in your meeting controls. Learn more about screen sharing.
2. Select the screen you want to share and select Optimize for video clip.
3. Click Share to begin sharing.
4. Start playing your video in full-screen while sharing your screen.
5. Ensure you are using the most stable version of the Zoom client

Optimizing for full-screen video while screen sharing

1. Click More in the screen share controls.
2. Click Optimize for video clip to enable.
3. Start playing your video while sharing your screen. 

Resources:
Sharing and playing a video

Optimizing a shared video clip in full screen