Simply put, AtoNs are navigational aids (lighthouses, markers, buoys etc.) that use the AIS system to broadcast their position and other information. Because they use standard AIS messages and frequencies, they can be picked up by your AIS receiver and be displayed on your chartplotter, radar or other AIS-enabled device.
This is how Wikipedia defines AtoNs:
"The AIS Aids to Navigation (AtoN) product standard was developed with the ability to broadcast the positions and names of objects other than vessels, such as navigational aid and marker positions and dynamic data reflecting the marker's environment (e.g., currents and climatic conditions).
These aids can be located on shore, such as in a lighthouse, or on water, platforms, or buoys. The U.S. Coast Guard has suggested that AIS might replace racon (radar beacons) currently used for electronic navigation aids. AtoN's enable authorities to remotely monitor the status of a buoy, such as the status of the lantern, as well as transmit live data from sensors (such as weather and sea state) located on the buoy back to vessels fitted with AIS transceivers or local authorities.
An AtoN will broadcast its position and Identity along with all the other information. The AtoN standard also permits the transmit of 'Virtual AtoN' positions whereby a single device may transmit messages with a 'false' position such that an AtoN marker appears on electronic charts, although a physical AtoN may not be present at that location."
sQuidd.io makes a global list of AtoNs available to OpenCPN users through its plugin. AtoNs are downloaded and displayed on OpenCPN charts by means of a gray sQuiddio marker, provided that the AtoN option in the plugin settings is checked (default is un-checked).
☞ IMPORTANT: AtoNs are provided on a BEST-EFFORT BASIS for CRUISE-PLANNING PURPOSES only. DO NOT RELY ON ATONS DISPLAYED BY THE SQUIDDIO PLUGIN FOR NAVIGATION PURPOSES, as the information retrieved from sQuidd.io, including position, operational status etc. may be out of date or incorrect.