Light, in matter, (air or water or glass, for example) has a velocity different from light in a vacuum, and different for different wavelengths (not very much, but some). Refraction, (bending) occurs when light passes from one material into another. The amount of bend depends on the velocity change and therefore on wavelength. This phenomenon can be used to make one type of spectrometer, or wavelength-measuring-device. That is the "prism" spectrometer.
Another type of spectrometer, the "diffraction grating", uses closely spaced parallel lines, on a mirror or glass plate, to bend the different wavelengths of light through different angles, thereby separating them.