What is the principle of Relativity?

The principle of relativity is not a new concept for us. We have seen that all framed of reference the move with uniform velocities with respect to an inertial frame are themselves inertial. Newton’s laws of motion are valid in the same form in all such frames. Standing on a railway platform, you can drop a stone in such a way that it hits your left foot. If you repeat the same experiment in the rain moving smoothly with a uniform velocity, the stone will again strike your left foot. One cannot distinguish between two inertial frames by repeating the same experiment in the two frames. No experiment done inside the train can tell whether the train is at rest at the platform or is moving at 120 km/h with respect to the platform provided there are no jerks, the train does not speed up or speed down and it does not bend.

This is the principle of relativity. There is not preferred inertial frame. All frames are equivalent. The motion between two frames is relative- you can chose any of the frames and call it at rest and the other in motion.

We can understand this on the basis of Newton’s laws of motion. These laws have the same form in all inertial frames. Whether you measure acceleration, force and mass on the platform or on the train, force always equals mass times acceleration. As the results of experiments are governed by Newton’s law, identical experiments will give identical results irrespective of the frame involved. But Newton’s laws govern only the experiments of mechanics! You can try to do an experiment related to electricity inside a train and tell if the train is moving or is at rest.

Is that it I've heard that there's a whole book on this topic and it's beyond the scope of a 12th student