Per some Googling:
The "high-grade" part means that my left rotator cuff is torn across more than 50% of its width.
The "partial" part means that it isn't torn all the way through its depth.
So the "high-grade" part is the bad news, the "partial" part is the not quite as bad news.
Next stop, orthopedist. Hopefully the next stop after that isn't surgery (which at least one article leads me to believe would entail three months with my arm in a sling and six months to maximum recovery -- that would make typing, etc. a real chore -- and which the testimony of relatives leads me to believe is unlikely to be full recovery).