There is actually two parts to the theory of relativity, the special theory of relativity and the general theory of relativity.
In the special theory of relativity Einstein explained the relationship between motion and time. It had worried physicists for a long time that light always travelled at the same speed regardless of the relative speed of the observer. Einstein realised that the only explanation for this is if time was slowed in relation to the observers speed as a fraction of the speed of light. The equation is to difficult to explain quickly.
In the general theory of relativity Einstein explains the connection between a mass and how it curves space time around it. Einstein realised that there is no difference between the inertia you feel in a lift or on a train and the force you feel as gravity. He showed that space time is warped by a massive object in such a way that a particle in the vicinity of a mass needs to move towards it in order to stand still in curved space time. The equations for the general theore of relativity are in 18 differential terms there is one term that needs to be put in from observation and the null term that makes no sense at all it is also partly in tensor. It is one of the most beautiful equations mankind has ever made.