The Dawn mapping spectrometer VIR is a Visual and Infrared Imaging Spectrometer that produces spectral images in the 0.25-1 µm and 1-5 µm ranges.
VIR is a rebuild of the VIRTIS mapping spectrometer on board the ESA Rosetta mission and it also derives much design heritage from the Cassini VIMS spectrometer. The design of the instrument fully accomplishes the mission's scientific and measurement objectives with a simple space-qualified low risk approach and high performance, reliability, heritage and cost effectiveness.
To know more about VIR, find here some technical details.

What is a spectrometer?

A spectrometer is an instrument that is designed to measure how much radiation of different "colors" is reflected or emitted by an object. The results of a measurement with a spectrometer are a spectrum, a graph showing how strongly the object reflected or emitted radiation of each measured color (or frequency)

Let's see an example:

The vertical axis measures "percent reflectance": something that reflects more light is lighter in color than something that reflects less light. In the part of the graph labeled "visible light", one of the rocks reflects a lot more red light than it reflects blue light. It's the red sandstone--that's why it looks red to us. But the andesite and basalt don't reflect any one color of light much more than any other color, so they appear dark gray or black.

Unfortunately, the color of a rock in visible light is not a very good source of information about what kinds of minerals are present in the rock. That's why many spectrometers measure reflected or emitted radiation in the infrared (infrared) part of the spectrum, in that part of the spectrum with a longer wavelength than visible light.
The graphs in the near infrared have many "dips" or "troughs" that can tell about the minerals that are present in the rock. For instance, the sharp troughs in the sandstone and limestone spectra are caused by the presence of water; these are sedimentary rocks, which formed under water. The smooth dips in the basalt and andesite spectra are caused by iron and magnesium. With clues like these, scientists can puzzle out the chemical and mineral makeup of a rock--which then become clues to the puzzle of the geologic history of the rock.

To know more, read about light , spectra and how a spectrometer works.