The skill stack is a concept created by Scott Adams. The idea is that you can be the best by layering (stacking) skills on top of each other, such that you possess a unique combination of talents. According to the theory, you don't need to be the best at any one skill, you just need to be better than average at a set of skills that are unusually combined.
Adams gives himself as the example. According to him, he is not the funniest guy, or the best drawer, or the best office worker, or the most dependable person. But he is above-average at all of those skills. The combination of unique talents makes him successful.
Adams gives Donald Trump as another example. Trump isn’t the best at insulting, or the most persuasive person, or the best at branding, or the best talent delegator, but he is above-average at all of those, the combination of which made him a very successful campaigner.
Key to this theory is that uniqueness is crucial. There are exponentially larger returns to being #1 at a certain activity than being #10. The problem is that it is extremely difficult to be the best in the world at a very narrow task, because it is likely to be fiercely competitive. Therefore, the way to succeed is to construct an arena suited to your unique combination strengths.
Most competitive endeavors are comprised of smaller skills. If a competitive endeavor does not exist that is tailored to your own combination of skills, create one. In other words, pick a game you can win.