Opencv calcHist and calcBackProject in plain english
See What is Back Projection in the openCV tutorials

An image histogram measures the distribution of colour (and brightness) of the pixels in an image.     

If you take an image and identify a region of interest eg. a hand, and calculate the histogram of the pixels in that object.       

Then take that histogram and a second image and essentially reverse the process - you pick the pixels in the second image that match the histogram from the first. It's this reverse process that gives it the name back-projection.        

You then make the assumption that areas of the image in the second image that have the same colour distribution as an object in the first image are an image of the same (or similar) object.