Put together by Einstein in 1915, the typical theory of relativity may be the modern theory of gravity. This is a very complicated mathematical theory which in a nutshell says that space and time, collectively referred to as spacetime, is curved through the existence of matter and, understanding that the motion of matter and energy is influenced by this curvature.
When Einstein devised his theory, he previously three experiments in your mind to try its predictions. One was the gravitational bending of starlight around the sun. Based on general relativity, the path of starlight which grazes the surface of the sun should bend by 1.75 arcseconds. This effect can result in measured within a total eclipse. In 1919, Sir Arthur Eddington measured precisely this bending throughout an expedition where he observed a total eclipse. This discovery made Einstein an immediate worldwide celebrity.
The second prediction may be the precession of planetary orbits concerning the sun. Based on general relativity, the perihelion with the orbit of mercury should precess about the sun by 43 arcseconds per century. This effect was in fact observed just before Einstein's theory and unexplained until predicted through the theory.
The 3rd experimentally verified prediction is the reducing of time because of gravity. This effect causes light to be redshifted inside the presence of a strong gravitational field, i.e. near the the surface of a massive star. This effect is observed in the spectra of massive stars. Taken to its extreme, this effect predicts the existence of black holes, i.e. objects that are so massive that nothing, not even light, can escape from their gravity. Black holes have been observed indirectly and today their existence is virtually undisputed.