An Non-directional Beacon (or NDB), is basically a radio transmitter that you can tune in, using a radio receiver. An NDB usually transmits morse code, indicating its identity but can also broadcast other things such as ATIS (Automatic Terminal Information Service) or other handy services. Most, if not all, of the ones you will find inside a flight simulator will broadcast a morse code identifier.
This is a companion discussion topic for the original entry at https://www.helisimmer.com/tutorials/finding-oil-platforms-using-the-adf-and-ndb/