What do spectrographs do to light entering it?

A prism can separate white light into the seven colors of the rainbow that humans are familiar with. White light is actually a combination of all the wavelengths of light.

The entire spectrum of light emitted by variations in the atomic quantum energy levels of helium is released when an electric current is applied to a tube of helium; these quantum energy levels are specific to helium.

The spectrograph will then provide an image of all the energy level changes in helium and the wavelengths of light associated with those energy levels by separating the light from each quantum energy change or jump.

Prior to its discovery in the earth's atmosphere, the element Helium could be identified in solar light thanks to a special spectrograph image. (hence the name Helium meaning from the sun)

Using spectrographs, light is broken down into its constituent wavelengths, and the intensity of each wavelength is measured.