This is to differentiate then from the ones that Newton used to make rainbows or the one in your survey instrument that makes the image appear right side up. They are called corner cubes because the three surfaces that make up the 'pyramid' are at 90 degree angles to each other and thus form the corner of a cube. As you have noticed, light entering the prism is reflected back to its source, even at relatively large angles of incidence. The angle of incidence is the angular difference between the incoming light rays and an imaginary line perpendicular to the face of the prism. A beam of light entering the prism bounces off all three back surfaces and exits at the same angle it entered.
Light also slows down when it enters glass.
Both of these factors make the distance measured longer than the distance to the front face or even the back of the corner cube. This distance is called the offset. This offset is a constant amount that the distance measured is corrected for a particular prism. Further complicating things is the fact that various manufacturers position the corner cube differently in the prism housing and the speed of light changes in different kinds of glass. This results in a multitude of prism offsets.