When you look at an object in motion, the frame of reference is different depending on where you are when looking at this object.
For example, if you are in a train you can say that from your frame of reference the ground outside is moving compared to you and you are not moving or the train is not moving. From someone on the outside of the train, they can say that the train is moving compared to the ground and they are not moving.
Another example could be if you are driving in a car. You can say that the road is moving backwards compared to you and you are stationary. Someone standing on the sidewalk can say you are moving compared to the ground. You can also say that you are not moving since you are stationary in the car.
If we draw the path of the van then we get a shape with two exposed points A and D. If we draw a line from point D perpendicular to BA we get point E. This gives us a right angled triangle ADE.
