Inertia frames of reference are those reference frames in which Newton’s laws are valid. They are non-accelerating frames (constant velocity frames).
If a person performed an experiment in a train, he will not observe any difference between the readings obtained when the train is moving at constant velocity or when the train is at rest.
Consider an airplane in flight, moving with a constant velocity. If a passenger in the airplane throws a ball straight up in the air, the passenger observes that the ball moves in a vertical path. The motion of the ball is precisely the same as it would be if the ball were thrown while at rest on Earth. The law of gravity and the equations of motion under constant acceleration are obeyed whether the airplane is at rest or in uniform motion.
Now consider the same experiment when viewed by another observer at rest on Earth. This stationary observer views the path of the ball in the plane to be a parabola and the ball has a velocity to the right equal to the velocity of the plane. Although the two observers disagree on the shape of the ball’s path, both agree that the motion of the ball obeys the law of gravity and Newton’s laws of motion and even agree on how long the ball is in the air.
Hence, we can say: There is no preferred frame of reference of describing the laws of mechanics.