White light consists of all the frequencies of electromagnetic radiation that are visible to us. In other words, white light is made up of all the colors of light. Objects that appear white do not absorb light of any color. Light of any color that falls on them is reflected back.
An object that has any other color absorbs light of all colors except its own color. For example a red object absorbs light of all colors except red; similarly, a blue object reflects only blue light, all the other colors are absorbed by it.
A black object appears black because it absorbs light of all colors that fall on it. Light of none of the colors falling on an ideal black object is reflected back by it.