That's not the spirit of the quote, though it's not entirely your fault. It's better reflected in the original, attributed to Aristotle: "Those who can, do; those who understand, teach."

There are a whole host of reasons someone may not be able to "do". Maybe they used to effectively "do", but got hurt, or just got too old. It could be the simple recognition that teaching is the "do", because of your deep understanding and ability to communicate and inspire.

It recognizes a conscious choice not to stubbornly attempt or continue to "do", but to instead take a back seat and direct your energy into others who will "do" more effectively than you ever could. It's about wisdom, self-awareness, and taking the long view. You are readying the next generation of do-ers, passing the torch, perhaps multiplying your efforts and extending your life's impact further into the future.