Out of all of Buffett's wisdom, I've come to realize that the single most important piece of advice he's given (for active investors) is to stay within your circle of competence.

The public markets are huge: you will never know everything about every company even in the S&P 500 and that's before small-caps and the rest of the world! Public markets mean that you don't have to have an opinion on every stock.

Staying in your circle of competence both means to not invest in companies that you don't understand, but also to understand the companies that you invest in! You must do your homework!

Through my relatively short investing career, insight or understanding sometimes only come after an extended period of reading and studying a company. It takes time and work to appreciate the nuances of an industry.

Early in my career, I would often get stuck on certain points that I can't resolve (i.e. why a company did X, or why an industry is y). It's more efficient just to mark it as an unknown and move on.