The first movie represents the motion of an object in space, far from the gravitational influence of large masses. The object moves in a straight line at constant speed (i.e., constant velocity).

The second movie shows the motion of the object in space, but the reference frame is accelerating. The motion is still at constant velocity, but the object appears to move in a curved path.

In the third movie, the object is in freefall in a gravitational field. There is no acceleration of the reference frame, but gravitation causes the object to follow the familiar curved trajectory it would experience on Earth.

The Equivalence Principle says that the effect of gravitation is the same as the the acceleration of the reference frame. In other words, the observer in the second movie has the same experience that the observer in the third movie has.

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