Oh wow, I didn't know what that meant either! So:
S.P.T. = Sybill Whatever Trelawney/ (Prophesizer)
A.P.W.B.D. = Albus Percivul Wilfred Brian Dumbledore (Prophesy-ee)
Dark Lord = Obviously Veldemort (Prophesy is about)
and ? Harry Potter = Harry Potter (included in Prophesy) ? stands for Neville Longbottom