Respond to the above how you will, but in my humble opinion, I think it's a friendly reminder to work outside of your comfort zone. To push yourself safely beyond levels where you've already found success. If you can curl ten pounds for hours, that's great, but you'll get much more out of your workout if you work up to those twelve pound weights.
From a different perspective, I think you can deduce that strength, be it physical or emotional strength, comes with believing in yourself. If you tell yourself "no," sell yourself short, or hold yourself back from doing that which you want to do...well, it's like a permanent stop sign on the road your life is on. Why do that to yourself? Stand tall. And in standing tall, you'll find new aspects of yourself, new levels of strength, that you might not known to have been there.
So...go. Find joy in your abilities. Forget about that which you're used to and remind yourself that there is more to this world...more to you...than you can possibly imagine. Move forward with eyes wide open. With the believe that life is yours for the taking.
That's how you grow. That's how you get stronger.
That's how you continue to write the book of you.