ORB Algorithm: What Is Intensity Centroid?
p and q - order of the moments, it is the analogue of the mechanical moments (i.e. of force or inertia). Image moments are very basic properties of image, invariant to rotation, could be used as simple descriptor.

Example,

Moment of inertia Ix= I2 = mass *x_coordinate^2.

Image moment Ix2 = intensity * *x_coordinate^2.

http://en.wikipedia.org/wiki/Moment_(physics)

http://en.wikipedia.org/wiki/Image_moment