The 10,000 hour rule is a cultural meme that has spread and found believers not because it is grounded in reality but because it is cognitively seductive. In the first place it should seem highly suspect that something as poorly-defined and multidimensional as "mastering" (how do you measure mastery?) a "field" (as if every field had the same complexity and breadth) could always be done by anyone (independently of the obvious issue of individual variations in intelligence and talent) under a constant, conveniently round figure of 10,000 hours of practice.
As a cultural meme it works in a similar way to the myth that "humans are only using 10% of their brains". I expect it to die a slow death over the next 50 years.
I think its popular because 10,000 hours seems nicely out of reach for most people, so it lets you off the hook for not being good at stuff. It's not my fault I can't play the piano: my lifestyle precludes it. If only I didn't have a job/kids/whatever, then people could see how creative I really am. Gladwell allows one to continue doing nothing while still imagining oneself as a thwarted hero. A great deal for just $16.99.
I'd always understood it as figurative for "a whole lot of time." I think it works as both an encouragement and a warning: if you think you can become a master in a few weeks, think again. But if you've been practicing for a year and you feel like you're never going to get there, keep at it! You're making real progress even if it doesn't seem like it.
Malcolm Gladwell is a tool, but the idea makes sense on some level because he borrowed it from other areas.
If you've been focused exclusively and passionately on a specific field of study for the equivalent of 5 full time years, you will be an expert/master at that thing. The scope or meaning of that mastery is going to depend on what it is. If you're a plumber, you're going to be adept at using your tools and be an expert in the codes for plumbing systems and maybe some specific sub-domain (like industrial cooling).
I use a blue collar example here because there is an actual process to be designated a "master" in a trade. Usually it requires a minimum of 5 years of practicing your craft at the journeyman level. (Along with training and alot of study) Assuming a 2,000 hour work year... that's pretty much your magical 10,000 hour benchmark.
As a cultural meme it works in a similar way to the myth that "humans are only using 10% of their brains". I expect it to die a slow death over the next 50 years.