Multiplication and Substraction

Notice how the length of the vector (visualised by the length of the line) has been multiplied by 0.5 (i.e. halved).
The fact that the vector got multiplied by 0.5 means that it grows half as fast as the mouse when you move the mouse to the right/left/up/down.

Key idea: the vector under observation is the one called mouselocation. Mouselocation vector origin is 0,0 and its default "goal" (the mouse position) is 0,0 hence you see a point. As the mouse moves and hences the "goal" moves.
Notice the translate. The translate shifts the point of reference 0,0 to 200,200 thus affecting everything in the draw code. BUT, it does not affect the mouse coordinates because the translate function is run after the code for the mouse coordinates. So we see through print that:
Mouse x/y and mouselocation vector's x/y are not the same value EVEN  THOUGH they change at the same rate*. You could say that between mousex/mousey and mouselocation vector, there is an invisible vector whose origin is mouse X/Y and whose target is mouselocation's coordinates. Hence, moving the mouse pushes the invisible vector which pushes mouselocation away from the mouse.

By substracting this invisible vector, mouselocation's x/y and mouseX/Y become one. Hence, the end of the line (the visualisation of the mouselocation vector) has* the mouse location.

*If you uncomment, the multiply method, you will see how the vector is "scaled" by half (i.e it gets shorter) when multiplied by 0.5