According to Google, radial velocity is "the velocity of a star or other body along the line of sight of an observer".

If the number is positive, does that mean it's moving away or towards us?

If you are referring to the Earth...

If the number is positive, the star is moving away from us. When the number is negative, the star is moving towards us.

Hope that helps!

• What do you mean by "us"? If you mean the Earth, then the answer is incorrect Aug 19, 2018 at 6:34
• I used the word us in the original question so I guess he was following suit. Your point below is noted. Aug 19, 2018 at 11:48
• Hmm... Makes sense. Thanks for pointing it out! Aug 19, 2018 at 15:03

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).