what does dispersion of white light mean?

Dispersion of white light is the phenomenon of splitting of white light into its constituent colours upon incidence on a highly refracting medium (prism) due to difference in their speeds of propagation in the refracting medium. A rainbow is formed when light is dispersed through raindrops.

However dispersion is not limited to only white light. It can be scattering of any light which contain more than one component of colour.