An astro is a typically working class person involved in a space industry such as planetary mining.
The term astro could also be applied to any crew member of a spaceship, but it generally isn't applied to officer class members. It probably traces its lineage from "astronaut". Another term used for lower class astros is "space bum" (Lister's self-titled occupation in "Justice" and "Psirens" and also referred to as such by Holly in "The Inquisitor").
Astros have been mentioned sporadically throughout the run of the television show, from the first to the tenth series. At least one astro made it three million years into Deep Space as Lister had. ("Psirens")