Only light has “absolute” motion
Consider this scenario: a train is approaching and you have to measure how fast it’s moving. If you know the length of the train, it’s simple. You measure how long it takes for the train to go past you, and you know that it covers its own length in that period of time, giving you a starting point for your calculations. However, if you were to move at half of the speed the of train in the same direction, it would take twice as long for the train to pass you. You might say that the train is still moving at the original speed and the slower speed is just relative to you, but it’s important to remember that the earth is moving around the sun at 67,000 miles per hour. Should you add that to the speed of the train too? Einstein realised that all motion is relative to who is taking the measurement, except for the motion of light. If you could travel at light-speed (somehow), and raced a beam of light, the result would depend on who was watching. If there was a third observer, he would see you and the light arrive at the same time, but you would see the light zip past you and make it to the observer first. Light is the only thing we know of with “absolute” motion, so it doesn’t apparently slow down if you’re moving in the same direction like a train would.
A crash course in quantum physics