A chip package that contains two ball grid array (BGA) chips.  A package on package (PoP) typically includes a RAM chip on top and a CPU chip on the bottom.  See 
BGA and 
chip package.
A 3D Package
The top BGA package typically contains RAM, which can also be a multi-die module itself, while the bottom BGA holds the logic chip (the processor).  The light green elements are the BGA substrates.  The PCB is the printed circuit board.