The zoom extent is the range of zoom levels a tileset is visible at. You can see the zoom extent of a tileset by selecting a tileset from the Tilesets page.
When a tileset is created in Mapbox Studio, a zoom extent is assigned to it to make sure that your data looks good. By rendering your tileset only at certain zoom levels, lower-resolution data will be limited from appearing at higher zoom levels, and detailed data won’t be cluttered on map at lower zoom levels. If you wish to change the zoom extent of a tileset, you can do so with other Mapbox tools.