I've followed Jonathan Clements writings for a long time going back to the days where he had a column in the Wall Street Journal. He always writes on important aspects about money and investing. Better still he avoids the "double speak" of the financial world and presents subjects in a clear, concise manner.
If someone really knows a subject, like Jonathan, they are able to easily explain complex material in simple terms where most everyone can comprehend the subject. Jonathan is just that person.
The link in this post explains a very important aspect of compounding that any serious investor should know through and through. The idea of building a portfolio is to get to the point where the amount a portfolio earns internally is greater than the amount the investor could contribute to it. This takes time, but once it takes hold, it becomes a self-sustaining process.
Check out the attached linked and understand it well. You'll be well on to your way to a well-funded retirement!