AIS receivers as the name suggests only receive AIS messages from other AIS equipped vessels.
This data is transmitted from boats that are equipped with Transceivers/Transponders, allowing you to know their location and heading. Nearly all commercial ships on the water are now required to transmit AIS data, this data includes the vessel name, course, speed and current navigation status.
An AIS receiver connects via NMEA to a compatible chart plotter and will add an AIS overlay displaying this data on your screen. A great tool for navigation as it makes you aware of the vessels around you, as well as their movement.
AIS receivers will also receive data from AIS MOB beacons, allowing you to locate and recover crew members or casualties that have fallen overboard that are carrying an AIS MOB beacon.