This comes straight from the Pirates themselves, as everyone that covers the team tweeted out something along these lines almost simultaneously:
#Pirates LHP Wandy Rodriguez has been given a PRP injection and will be on a rest, treatment and rehab plan for three weeks, per team.
— Michael Sanserino (@msanserino) July 1, 2013
PRP stands for platelet-rich plasma. To roll it back a bit further, plasma is the liquid part of your blood (that is, the liquid that carries your blood cells all over your body) and platelets are blood cells that are rich in growth factors. Basically, PRP injections are intended to speed healing in situations involving tissue damage, which you might find in, say, a pitcher's ulnar collateral ligament.
Recently, there's been a pretty well-established pattern of pitchers who've had elbow or forearm problems followed by rest, then PRP-injections, then Tommy John surgery. Ben Lindbergh at Baseball Prospectus investigated just that ov...