A navigational aid (NAVAID), also known as an aid to navigation (ATON), is any type of signal, marker, or guidance device that aids the traveler in navigation, usually in maritime or aviation. Common types of such aids include lighthouses, buoys, fog signals, and day signals.
This is how to find them in the FleetMon Explorer:
Open the Vessel Filters menu on the right
Select "Aid to Navigation" as Class
AtoNs are marked with this symbol on the map: