Parcel boundary lines show up on the map by default. If you want to hide those for a cleaner view, you can do so through Base Maps.
Find the main menu on the left hand side of the map. Click on Base Maps (the top option). At the top of that section, find the Eyeball Icon and click it to toggle the lines on and off.
Note: If you have a parcel or parcels selected, those will remain highlighted on the map.
