Well, you may consider that you and your friend are sitting in two cars, say two Camaros, and both are driving at the same exact speed side-by-side on a long expressway. Both of you down your glasses and look at each other. You will see each other to be stationery and even the pair of die on your back view mirror to be stationery but the trees on the expressway banks to move backwards. Now if another friend of yours comes riding a Agera or Aventador and smokes you he will see you ride backwards and you will see him move ahead while your Camaro as stationary. This is what is the "Theory Of Relativity", in simple terms! Motion of any other physical factor of a body cannot be measured without using a relative term to express it.