Produced by Einstein in 1915, the typical theory of relativity is the modern theory of gravity. This can be a very complicated mathematical theory which in summary says that space and time, collectively known as spacetime, is curved through the existence of matter and, and 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 test its predictions. One was the gravitational bending of starlight across the sun. In accordance with general relativity, the trail of starlight which grazes the top of sun should bend by 1.75 arcseconds. This result can be measured within a total eclipse. In 1919, Sir Arthur Eddington measured precisely this bending in an expedition in which he observed a total eclipse. This discovery made Einstein an instantaneous worldwide celebrity.
The second prediction will be the precession of planetary orbits about the sun. According to general relativity, the perihelion of the orbit of mercury should precess concerning the sun by 43 arcseconds per century. This effect was in fact observed prior to Einstein's theory and unexplained until predicted from the theory.
The 3rd experimentally verified prediction is the slowing down of energy due to gravity. This effect causes light to be redshifted within the existence of a strong gravitational field, i.e. near the the surface of a huge star. This effect is noticed in the spectra of massive stars. Come to its extreme, this effect predicts the presence of black holes, i.e. objects that are so massive that nothing, not light, can escape from their gravity. Black holes happen to be observed indirectly now their existence is practically undisputed.
