Answer:
The concept of reference frames was first introduced to discuss relative motion in one or more dimensions. When we say an object has a certain velocity, then this velocity is with respect to some frame that is known as the reference frame. In everyday life, when we measure the velocity of an object, the reference frame is taken to be the ground or the earth.
For example, if you are travelling in a train and the train is moving at a speed of 100 km/hr, then your speed according to another passenger sitting on that train is zero. According to him, you are not moving. But if someone observes you from outside the train, standing on the ground, according to him, you are moving with 100 km/hr as you are on the train and the train is moving with 100 km/hr.
Here, the motion observed by the observer depends on the location (frame) of the observer. This type of motion is called relative motion.