The theory of relativity was put forth by Albert Einstein in two parts. The first part called the special relativity theory was published in 1905. It stipulates that the velocity of light in vacuum is constant irrespective of the relative velocity of the source and the observer. As a consequence, the measurement of length is dependent upon the relative velocities of the measuring rod and the observer.
For example, to a stationary observer on a railway platform, a train moving with a speed approaching that of light appears shrunk lengthwise. At the same time, to a passenger looking through the window on the train, the railway platform and the observer appear shrunk. But, to both the observer on the platform and the train passenger the immediate surroundings appear normal.
The second part of this theory published in 1915 is called the general relativity theory. It states that the gravitational attraction between two objects is not due to an attractive force but because of a curvature produced in space-time continuum by mass.