This Vuforia Knowledge database article explains in detail the meaning of the pose matrix, you should probably have a look at it.
To make it short, the pose matrix is a 3x4 matrix whose last column is the translation vector <x,y,z>
from the camera to the detected target. The "closest target to the center of the screen" should thus be the one with the smallest <x,y>
vector.
Hope this helps!