A light wave is just like the wave you would see at a beach. Part of it moves up and down. Sometimes, the waves have peaks that are close together. And sometimes, the waves have peaks that are far apart. This happens with light too except light waves are very very tiny.
Some light waves have peaks that are closer together than other light waves.
The color of light depends on the size of the wave. If the peaks are further apart, the color looks more red. And if the peaks of the light wave are closer together, the light looks more blue.