LED grow light
All green plants require light to perform photosythesis, during which special cells convert the energy contained in the light into sugars. In the process, plants give off oxygen.
Plants vary widely in how much light they need to thrive — from low-light houseplants, such as dracaenas, to fruiting edibles, such as tomatoes, that need much brighter light.
Seedlings require lots of bright light, and when they don't receive enough they get weak and leggy. In most cases, even the sunniest windowsill won't provide the intensity or duration of light they need, especially in winter.