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

The radial velocity of Sirius according to Google is    −5.50 km/s. 

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!

Source with more info: https://cseligman.com/text/stars/radialvelocity.htm