Generally, dispersion is an effect that happens because the velocity of waves in a material depends on wavelength. This manifests in many different phenomenon, but only one is encountered on a regular basis: the index of refraction is dependent on wavelength. This means different wavelengths are refracted at different angles. This is used beneficially in something like a prism, where it separates the wavelengths of light. This shows up detrimentally as chromatic aberrations.
