It means "best regards". In Morse code transmissions, a lot of words and phrases are shortened to commonly understood abbreviations and pro-signs. You can see an example here: http://lidscw.org/resources/cq-qso-template and a list of such abbreviations here: https://cwops.org/wp-content/uploads/2018/07/CW-Abbreviation...

It's a short form of "best regards". In message handling there was a table of numbers that could stand in for phrases to save time. That one made it to common ham lingo.

"Best regards"

As I understand it, '73' comes from the landline Morse/news wire reporting tradition.

Thanks all!