principle of relativity
The principle of relativity says that the laws of physics are exactly the same in every inertial frame. Whether you are at rest or gliding along at a steady speed, the rules governing balls, springs, magnets, and light do not change. There is no preferred frame that nature secretly regards as 'truly at rest'.
A vivid consequence is that no experiment performed inside a sealed, smoothly moving cabin can tell you whether you are moving or standing still. Pour a drink, bounce a ball, watch a flame — everything behaves just as it would on the ground. Only by peeking outside, at something else, can you detect relative motion; absolute uniform motion is undetectable in principle.
Galileo first dramatised this idea with a ship's cabin below decks, and Einstein took it as one of the two pillars of special relativity. Einstein's bold extra step was to insist the principle covers electromagnetism and light too — which forced the speed of light to be the same for everyone and overturned the old notions of absolute time and simultaneity.
Sealed, smooth, steady motion is undetectable from the inside — only relative motion can be seen.
The principle applies only to uniform (non-accelerating) motion. Acceleration and rotation can be felt from inside, which is why a braking train or a spinning carousel does not obey it.