A magnetometer is an instrument to measure the strength of a magnetic field. There are many kinds now in use, both on Earth, e.g., for geology, and in space probes. The first type invented used the fact that a voltage could be induced by using a conductor in the magnetic field (Hall effect). In space missions, often they use magnetic induction to sense a field's change (as the probe traverses a magnetic field), often with three elements aligned to record the changes sensed in the three dimensions.
Magnetometers are used to study the solar magnetic field as well as that of planets and moons visited by space probes. An overall dipole magnetic field suggests a dynamo-induced magnetic field, and a change in the magnetic field over time suggests movement of something conductive within the planet or its atmosphere, making it a probe of geological activity, or ocean or atmosphere activity.