Illumination, required for photosynthesis.
Virtually all plants require light to power the chemical reaction known as photosynthesis, to produce sugars and cellulose (exceptions include some rare orchids such as Hexalectris which are at best partially photosynthetic).
Different plants have adapted to require different amounts of light. For example, trees that grow at high latitudes are generally adapted for poor light conditions, and low latitude desert plants are adapted for high light conditions.
Use this tag for all questions about the required light levels for your plants. Use it with tag(s) specific to the plants in the question.