What does the 10,000 hours rule mean in Outliers by Malcolm Gladwell?

Malcolm Gladwell refers to the 10,000 hour rule in his book Outliers in an attempt to put a number on the preparation and practice one needs in order to become a true expert. Gladwell explains that preparation and talent are needed for exemplary achievement, and to excel at a complicated activity of any kind, hours and hours (a minimum of 10,000, to be specific) of practice must be invested.
To support this figure, Gladwell draws on the 1973 study on expertise conducted by Herbert Simon and William Chase. In this study, Simon and Chase determined that a chess master must spend at least 10,000 hours practicing and playing chess before becoming a master of the game. Gladwell expands on this point in order to illuminate his position that talented individuals who have made a name for themselves have rarely done so without putting a lot of time and effort into the sharpening of their skills. According to Gladwell, "the closer psychologists look at the careers of the gifted, the smaller the role innate talent seems to play and the bigger the role preparation seems to play."

In his nonfiction book Outliers: The Story of Success (2008), Malcolm Gladwell investigates the qualities that successful people have in common, analyzes the various factors that contribute to success, and and suggests strategies for anyone (for example, the reader) to find more success in his or her life.
One of the key factors and strategies that Gladwell writes about in the book is the "10,000-Hour Rule." The concept of this rule is that for any person to become an expert in anything, he or she must spend 10,000 hours practicing it. Those 10,000 hours must be spent in a correct, rigorous, and focused way: Gladwell calls this, "deliberate practice."
Gladwell gives examples of famous people who have employed the 10,000-Hour Rule to achieve their goals. Dedication to "deliberate practice," he writes, helped the Beatles become world-famous (they played for a long time together before they became well-known internationally), and it helped Bill Gates (who spent many hours on his computer as a teenager and young man) make great technological advancements. 
Gladwell's 10,000-Hour Rule was based in part on a 1993 study by Anders Ericsson, Ralf Krampe, and Clemens Tesch-Römer. The trio observed violin students at a music academy in Berlin, noting that the best musicians had clocked an average of 10,000 hours of practice by the age of twenty. After Outliers was published, Ericsson co-wrote a book on the science of expertise. In it, he addressed some of the flaws or misconceptions about the 10,000-Hour Rule:

The rule is irresistibly appealing. It's easy to remember, for one thing. It would've been far less effective if those violinists had put in, say, eleven thousand hours of practice by the time they were twenty. And it satisfies the human desire to discover a simple cause-and-effect relationship: just put in ten thousand hours of practice at anything, and you will become a master. . . . [but] there is nothing special or magical about ten thousand hours.

The number, they point out, is just an average of how much their students practiced. Gladwell responded to the criticism by slightly clarifying his concept after Outliers was published:

There is a lot of confusion about the 10,000 rule that I talk about in Outliers. It doesn't apply to sports. And practice isn't a SUFFICIENT condition for success. I could play chess for 100 years and I'll never be a grand master. The point is simply that natural ability requires a huge investment of time in order to be made manifest. Unfortunately, sometimes complex ideas get oversimplified in translation.