Imagine you are travelling in a moving train. Do you observe any change in your position with respect to your co-passengers? Is there any change of scene you view through the window? The change of scene indicates that the train is moving. That is, an object is said to be in motion if it changes its position with respect to its surroundings in a given time.
We know that a table in a room is at rest i.e., its position with respect to the walls of the room does not change with time.
Those who are fond of watching night sky would have observed that the position of stars and planets change whereas we are not moving. But in reality the earth is also moving and so also all the objects on the surface of the earth. Thus, an object which appears to be at rest, may actually be in motion. Therefore, motion and rest are relative terms. Hence, to describe the motion of an object we have to specify how its position changes with respect to a fixed point called the “origin”.
We cannot tell whether an object is in motion unless we have a frame of reference. A frame of reference is another object or scene with respect to which we compare an object's position.
Example for frame of reference
Look at the figures. In fig.1, the car is to the right of the tree. In fig.2, after 2 seconds, the car is to the left of the tree. As the tree does not move, the car must have moved from one place to another. Therefore, here the tree is considered as the frame of reference.
