The verb "to apprehend" has two primary uses: 1) To seize or take control of something/someone physically. "We have apprehended the criminal, and are taking him to jail." 2) To have an understanding of something. "Thanks to the teacher's explanation, I have apprehended the meaning of the book." (A third meaning is to be 'waiting in fear' but I rarely hear it actually used this way outside of older novels.) The difference in definitions is not terribly different if you think of gaining understanding as "seizing understanding", "taking understanding" or "grasping understanding" If you think of the first definition in an abstract way and apply it to thought, it is easy to see how one can come up with the idea of "apprehending knowledge" being a form of understanding. Hope this helps :)