What preserving actually means
Look at how the character is built. 宀 on top, a roof. 寸 underneath, a hand. A hand keeping something under cover. That is the whole idea before a single word of philosophy. 守 (Shǒu) means preserving, keeping, guarding: not gripping a thing tightly, but keeping it from being carried off by everything else.
Chapter 52 of the Tao Te Ching gives the reason, and it is almost uncomfortably current for a text roughly two and a half thousand years old. Block the openings, shut the gates, and all your life you will not be worn out. Open the openings, add to your affairs, and all your life you will be beyond help. Exhaustion, in this reading, is not caused by effort. It is caused by openness. Every opening left open is an input you did not choose.
The image is a window with wooden shutters. An opening you can close, and choose when to open. Shutters closed do not make a room dark: daylight still comes through the slats. They only stop everything from arriving at once. A door that never closes is not a door. It is a hole in the wall. The ability to close is what makes openness mean anything.
It sits close to two neighbors and is neither. 靜 is what happens inside once you stop stirring; 守 is the door that lets it happen. One is the stillness, the other is the boundary that protects it. And 和 was about staying in the room with people who differ from you. 守 is about the window: what comes through it, and when. Nobody gets shut out. The noise does.
Where Lao Tzu speaks of it
The cost of having no edge
Nobody is exhausted by one thing. It is the number of doors left open. Every channel that can reach you at eleven at night, every alert that was never switched off, every feed that refreshes itself. The day starts to feel like it happened to you rather than being lived by you, and the tiredness has nothing to do with how hard you worked.
There are two easy misreadings of the fix, and 守 is neither. The first is isolation dressed as self-care: cut everyone off, nobody is owed access to you. Chapter 52 is not about relationships. Its warning is against 濟其事, adding to your affairs, letting every input through. The second is productivity advice in a robe: guard your attention so you can output more. The verse gives its own reason, and it is simpler. So that you are not worn out.
守 is doors with hours. A first hour that nothing gets into. One window open at a time. Being reachable without being reachable constantly. The people you let in on purpose get more of you, not less, because you are not already spent on everything that simply walked through.
Choose one opening to close for the first hour of tomorrow. Not everything, just one: a notification, an inbox, a feed. Leave it shut until the hour is over, then open it on purpose rather than by default. Notice that nothing needed you as urgently as it seemed.