What does it mean to be a "master" of a certain trade?

In most professions you listed, there are highly specific requirements and training that have to be met. And you have to pass several tests.

For example, see this which describes the difference between a journeyman electrician and a master electrician.

I don’t know as much about plumbing, but I imagine there is a similar set of requirements.

In a way, this parallels the Middle Ages where there were guilds and apprenticeships that prepared the next generation of masons. (see Ken Follet’s novel Pillars of the Earth). Except that now, the requirements are more regulated and the examinations more standard.

https://www.hallettmarble.com.au/what-are-the-qualifications-of-a-master-stonemason/

https://everythingwhat.com/what-is-a-master-pipefitter

and so on

It’s just the trade lingo for what career level someone has attained within the trade like apprentice, journeyman, master. Sometimes it’s actually part of licensing. It’s similar to an administrative Level A, B or C or Professional level II, Associate professor, Principle scientist, professional engineer, engineer in training etc… I think to be a “master” usually means you’re like the foreman running the show or running the company. I don’t think a journeyman can run their own company in certain trades.

Many times a person has to complete 4 years of apprenticeship training to be a “master.”

I dated a Masterbaker once. 8^) {{groan}}

No one made the joke about baiting fish hooks!

I came close, but that one didn’t occur to me. ;o)

Like a master charter fishing mate?