The plant grows gangly or stalky long white shoots, struggling to find some light somewhere. You may have seen it happen to outdoor plants that you overwinter indoors.
For culinary purposes, it’s used more to mean blanching the plant — growing them in the dark so that they stay white. Chlorophyll is what makes plants green; it can also be what makes some plants bitter and inedible. The blanched plants taste milder; and sometimes are also more tender and less fibrous. These vegetables are said to be “etiolated.”
It’s usually done in the spring. The plants are planted, usually indoors, after the winter. The building that they are being grown in has its temperature slowly increased to signal to the plants that it’s spring and time to start growing. They do, but they don’t develop chlorophyll because they can’t without any light.
This is done to seakale, champagne rhubarb, chicory, etc.
It is possible to blanch vegetables outside as well by putting pots over top them.
The verb, etiolate, comes from the French word étioler, which may come from an old French word for straw, esteule.