Check out this video of Mavis Co-Founder, Dr Phil, as he dives into the Monitor Tools (including Zebras) inside Mavis Camera:
The zebras visualisation is a tool to assist with exposure. Pixels over a set luminance threshold level are filled with a striped or cross-hatch pattern (commonly known as a zebra pattern) to dramatically highlight the areas that are over the given exposure threshold.
The threshold value can be set by tap and holding down the Zebras icon. You can use a high threshold level to make sure your overall scene is correctly exposed or use a lower threshold (e.g. 70-80%) to view highlights across a range of skin tones.
Additional options can also be accessed by tap holding down the Zebras icon within monitor toolbar or via the settings menu (Settings > Viewfinder > Zebras).