There are charts spread out evenly and there are charts with a magnet: a zone where several planets crowd together. That is called a stellium, and when you have one, it marks a central theme of your life impossible to ignore.
What it is, exactly
A stellium is the gathering of three or more planets in the same sign or in the same house. That concentration makes the energy of that sign, or the area of that house, very dominant. It is like having the volume of one part of your personality turned up far above the rest.
What it means to have one
A stellium usually indicates a zone of intensity, talent and sometimes obsession. If you have it in a sign, that sign defines you far more than usual, even if it is not your sun sign. If you have it in a house, that area of life (love, work, home) becomes an axis around which everything else revolves.
A stellium concentrates your strength. The art is not letting it overwhelm you: channeling it instead of being governed by it.
The challenge of the stellium is balance. With so much energy in one point, other areas can fall into the background, and sometimes it is hard to see beyond that focus. But well integrated, a stellium is an enormous source of mastery: it is where you can become extraordinary.