As others have mentioned, 73 means "best regards" in hamspeak.

It comes from the days of the telegraph, when a group of numbers were
used as a sort of shorthand for common phrases and functions. The number
73 meant that the signature immediately followed, sort of like:

73,
Roy Lewallen, W7EL

A lot of hams like to say "73's" which of course means "best regardses",
an endearing and Gollum-like affectation. These are, I suspect, the same
general group that go to ATM machines, use TTL logic, and watch LCD
displays. . .