Explain how an observer uses a frame of reference to detect motion?

This is what we call relativity. As we being on the platform observe one sitting in a train at rest throwing a ball vertically upward and catch it back as it comes downward, we see the ball is going along a straight line.

Now the same phenomenon is observed by us when the train is at uniform speed. We still stand on the platform which is at rest. But the man in the train is moving at uniform speed.

Now for him he experiences the same straight line movement of the ball up and down. But for us being on the platform the ball seems going along a parabolic path.

This is what we call relativity. Our frame of reference is the platform at rest. But the man in the train has his frame of reference moving at uniform speed.