We all have limits. They're far higher and further than most people think. They can be pushed and developed with time. But we have limits. The person who truly pushes themselves in fitness, business, career, or anything else will eventually run into the wall, and see they need more training.
Most people do not need to worry much about their limits in the short-term; they're nowhere near them. Learn the basic safety and protocol for what you're doing, and then just go do it. Most people get nowhere near their limits.
But, if you're a Type-A type who wants to push yourself, recognizing and working with your limits is incredibly important. If you're really exhausted, dialing back your activity to something reasonable can be better than going to breaking point.
There's no shame at all in cycling off a weight training program in favor of a low-key week of stretching and recovery. Taking a few days off to sleep enough, plan, and meditate is good. No matter how hardcore you are, eventually you might need to cancel a few scheduled appointments in order to recover and keep your edge over the long haul.
These are all smart and necessary. In the long term, it means building rejuvenation in and getting an intuitive grasp of when you're about to run face-first into the wall. Sometimes you want to do that -- it's good to learn where the limits are -- but stopping yourself from getting burnt-out, injured, or stuck on the hamster wheel is key.