It’s a concept that is used to describe the amount of resources available for the amount of children that you have.
The theory is based on the idea that parents only have a limited amount of resources. With each additional child the parents resources get diluted. Imagine two families with the same amount of resources, one family only has one child and the other family has four. The family with one child has four times the amount of resources available for their one child than the other family.
This same idea is true for projects, friends and just life in general. The more you do the more spread out you are, the less energy, time, and resources you have for each addition to your life.
The common pattern is that less is more. Don’t fall into the trap of doing everything. You’ll end up with nothing.
The exception in my opinion is when a person has conquered a project to the point where people and their environment can build up on it or when a person is questioning what they truly want to do with their life. For example, conquering a project to benefit mankind and nature could be feeding and growing the soil to cultivate food for us and the microbes in the soil, win win. Next steps in succession could be selling the extra produce, giving it away, or making value added products like making pickled vegetables or raising farm animals. Questioning what you want to do is simply trying a lot of things to find the few things that you are passionate about, running through the gauntlet.
All of this is slow and small solutions in permaculture or the Tortoise and the Hare if you like fables.