Here Goes:
If you point your camera at something with distinct verticals (e.g. a tower block) and keep the focal plane vertical (i.e. don't point the camera up or down) the sides of verticals will 'stay' parallel.

Now incline the camera upwards (like you did in your image ) the vertical features will appear to lean together as they go from bottom to top. Incline the camera downwards and the opposite happens, they appear to lean together from top to bottom.

Think of the shape of a keystone in a stone archway to see where the terminology comes from, and given your IT background, the shape of a projected screen if you raise the front of a projector (if it doesn't have automatic keystoning rectification.)

HTH