When white light is refracted by a glass prism, it is refracted at the air-glass interface as it enters, and again as it leaves the prism. White light is made up of many different wavelengths, each wavelength corresponding to a different colour. The amount by which a ray is refracted depends on the wavelength. Red light - the longest wavelength of visible light - is refracted the least, and violet light - the shortest wavelength - is refracted the most.
White light refracted into many different colours is said to have been dispersed, and light so dispersed can be reformed into white light by an appropriately placed prism.