Can someone please explain to me what a Blood Traitor is???

I'm pretty sure that "Blood Traitor" just means a pure blood (or half blood, I'd assume) that assosiates with Muggles and/or Muggle-borns. It's basically just means that they're "betraying" their pure blood status por sympathizing with "lesser" people.

A blood traitor is someone who is pure-blooded but (apparently) doesn't act like it. For example the Weasleys associate with muggles, Tonks' mother, Andromedia, married a muggle born so she was disowned.
They're one of the many classes that the Death Eaters don't like.

A blood traitor is a pure-blood who marries a mud-blood or a muggle.

well blood traitor means those pure blood wizards who are friends with Muggle_borns and other low standard beings....

Well it should be about the family in which he or she belongs which has marrid too many mugelborns. as u said ron's mam and dad are what they call pure blood but their family should be having lots of mugelborne or mugel members

as they said iTS A DISGRACE.Ima proud pureblood and a faithful death eater!