When heart issues are discussed, the letters 'PVC' usually stand for Premature Ventricular Contraction. It's a heartbeat that comes a bit early. Most of the time, these beats are benign, and we all have them occasionally.
As far as I know, there is no such thing as Borderline PVC; you either have these beats or you don't. You can google the phrase to see if it means anything else, but you may just have mis-heard or misread something.