A Mass Spectrometer (MS) is an analytical instrument that can identify the atoms or molecules in complex chemical substances by determining their molecular mass. The Mass Spectrometer onboard the VSSEC Mars Rover is designed to identify a wide range of organic (carbon-containing) compounds.
Mass Spectrometers use extremely small samples, and can detect very low concentrations in chemically complex mixtures. They are used in forensic science, sport (to detect drugs in urine samples), medicine and space research.
Read more about how a mass spectrometer works at Explain That Stuff.
The smallest mass spectrometer ever produced was used by astronauts on the International Space Station. It is only 5 cm long and weighs 2.3 kg.
A mass spectrum will usually be presented as a vertical bar graph, and can be identified by comparing it with known substances.