If you want to measure how badly a crystal is distorted around a dislocation, one neat trick is to draw a closed loop of equal steps around the dislocation line in the real, defective crystal, then draw the same sequence of steps in a perfect crystal with no defect. In the perfect crystal that loop will not close up on its own, and the extra vector needed to close it is exactly the Burgers vector. It captures both how large the slip displacement is and which direction it points in, which is why it is described as the magnitude and direction of the lattice distortion carried by the dislocation. It has nothing to do with grain boundaries or overall crystal orientation, and the slip plane normal is a separate quantity found from the cross product of the line direction with this same vector. So the Burgers vector represents the magnitude and direction of lattice distortion, option (B).