This is the book that popularized the 10.000 hours idea. The main idea behind the rule is this: There exists a level of mastery that can be achieved for any kind of subject, after having deliberately practiced and studied it for at least 10.000 hours.
10.000 hours means that with a full work week focus (40 hours) on something it would take you around 250 weeks to gain mastery in a subject. That's around 5 years of dedicated practice every day of the work week, every week, without pause for 5 years.
Obviously this estimate is a little flawed, because the capacity for focused practice is relatively limited (at least for most people), and hence the real time it takes most people to gain mastery in something is more like 10 or 15 years.
However, if something is pursued with such passion, for such a long time, the brain rewires a little and one can reach greater heights. One can become an "Outlier" as Malcolm Gladwell would call it.
But. There is another important component to success, and that's really what the book is about. While putting in the effort and practice is a necessity to become successful, you are not the only person willing to do so and hence you compete with others for success.
The problem now is that factors outside of your control (your genes, your upcoming, the people you randomly meet, your psychology etc.) affect your chances of winning at playing this game for success. Personal ability is only part of the story. The other part is determined by a lot of lucky coincidences.
Furthermore, once people get ahead in the game for success, they get more opportunities, more coaching, more training, better education etc. which means that small changes in circumstances due to chance get amplified wildly in the end.
Malcolm Gladwell props these ideas up with a few stories. The most memorable to me is that of how pro ice-hockey players are overwhelmingly born in January (or close to it), because of the cutoff dates for the year. Younger players are less able physically, since their bodies had less time to grow until then, hence older players (those born in January) have better chances of winning competitions and performing than younger players. This means they get more playtime, more practice and so their likelyhood at success in the game of ice-hockey increases dramatically, to the point where almost everybody in the pro ice-hockey scene is born around that time.
Similar dynamics exist for almost all other sports, music and pretty much anything where humans compete against each other.
Another thing is where you have been born. Not just for the dynamics of wealth at play, but also for the cultural norms that subtly influence your behavior and what you are likely to do in a given situation. The main thing is, the deeper you dig into success, the more weird patterns you start to see, and find, which show you that it's not just about the hard work being put in. And that's the main valuable lesson to take away from this book.
All this being said, even though a lot of success is up to luck, it's still a necessary requirement to put in the work and make an effort to become better than you were yesterday, otherwise the competition for success is lost before it even began.