Lots of context for this concept. You might be “at capacity” in any of a variety of ways.
At capacity in learning, a sort of plateau of absorbing new things;
At capacity with activities, not able to fit another event on your schedule on a given day;
At capacity in repetitions, not able to do another pull up or arm curl or plank;
At capacity with projects, every day already full with no room for another thing;
At capacity eating your vegetables at dinner; this affliction happens on nearly a nightly basis at my house.
You get the idea. But here’s the thing.
Capacity is a dynamic quality. It’s a parameter that must always be considered. Capacity might be limiting in the immediate, in the short term.
But in the aggregate, it’s just another variable that must be considered in striving for optimal outcome.
You should always consider capacity, and you should also test its limits. Carefully, thoughtfully, but for sure, test, push, strive to expand the limit.
It will make you better. It will help you reach your goals. It will make things GOOD.