An object is said to be motion if its position changes with respect to its surroundings in a given time. On the other hand if the position of the object does not change with respect to its surroundings it is said to be in motion.
An object in one situation can be at rest but in another situation the same object can be in motion.
For example: Consider a person sitting in a moving train. The person sitting in a moving train is at rest with respect to the other passengers but he is in motion with respect to the objects outside the train. Similarly someone sitting at house is at rest with respect to earth but it is in motion with respect to other planets. Hence it is said that rest and motion are relative. Motion of an object is always said relative to an observer or something at rest.
The position of a moving object goes on changing with respect to time. Hence to specify the position of the object we need to use a reference point and a system having a set of axes. The widely used and convenient system is the rectangular coordinate system consisting of three mutually perpendicular axes(x,y,z). The point of intersection of these three axes is called the origin which serves as the reference point. The position of the object at a given instant of time can be described in terms of position coordinates(x,y,z) i.e the distance of the position along the x,y and z axis.