The plot shows the type of data the magnetometer measures. These data were taken during Saturn orbit insertion.
Cassini is measuring the three components of the magnetic field (the strength of the magnetic field in three different directions) along with the magnitude of the magnetic field.
Visible is the bow shock (S1-S7), a shock wave surrounding Saturn's magnetosphere where the magnetic field increases abruptly, and the magnetopause (MP1-MP3), the boundary of the magnetosphere. There are multiple crossings of these boundaries because scientists think that the magnetosphere "breathes" in and out in response to changing conditions in the solar wind. The solar wind can be thought of as a fluid.
In the solar wind the field magnitude (bottom panel) is less than 1 nano Tesla (a unit of magnetic field strength). As Cassini gets close to Saturn, the field increases. At closest approach it was almost 10,000 times that value.