The number -5.5 km/s in this context means that the relative line of sight velocity between Sirius and the Sun is 5.5 km/s in the sense that Sirius and the Sun are getting closer to each other.
Note that the line of sight velocity of Sirius as observed from the Earth will vary throughout the year, since the Earth moves with an orbital speed of 30 km/s around the Sun. Astronomical radial velocities that are quoted in catalogues will almost certainly be corrected for this and will be referred to as "heliocentric radial velocities" if corrected to the Sun;s reference frame, or "barycentric radial velocities" if corrected to the centre of mass of the solar system (which is almost the same to within 10 m/s or so).