So white light is a mixture of all colours. That was easy.
Some physicists pretend that light consists of tiny particles which they call photons.
The speed of light is about 300,000,000 meters per second. When they hit something they might bounce off, go right through or get absorbed. What happens depends a bit on how much energy they have. These poor old physicists get a little bit confused when they try to explain why some photons go through a leaf, some are reflected, and some are absorbed. They say that it is because they have different amounts of energy.