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How does the constant speed of light relate to Einstein’s theories of relativity?

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gravitational lens — gravity A galactic cluster, about five billion light-years away, produces a tremendous gravitational field that “bends” light around it. This lens produces multiple copies of a blue galaxy about twice as distant. Four images are visible in a circle surrounding the lens; a fifth is visible near the center of the picture, which was taken by the Hubble Space Telescope.

The constancy of the speed of light is a fundamental postulate of Albert Einstein’s special theory of relativity. Einstein established that the speed of light in a vacuum is the same for all inertial observers, regardless of their motion relative to the light source.12

This principle contradicts classical Newtonian physics, which assumes absolute space and time and follows the law of addition of velocities. To reconcile the constant speed of light with the laws of physics, Einstein concluded that space and time are not absolute but relative to the observer's state of motion. This led to several critical consequences:31

Relativity of Simultaneity: Events that appear simultaneous to one observer may not be for another.3
Time Dilation and Length Contraction: Time runs more slowly and lengths contract in a moving body as seen by an outside observer.3
Space-Time: Time and space were merged into a single four-dimensional structure known as space-time.4

In general relativity, Einstein extended these concepts, proposing that gravity is not a force but the curvature of space-time caused by mass and energy.42