The theory of relativity is a mathematical description of a number of physical phenomena,
particularly regarding what happens at high velocities or in strong gravitational fields.
The special theory of relativity describes what happens when two objects (e.g. persons) move in relation to each other,
for example that one's time is slower compared to the other's, and vice versa.
The general theory of relativity extends the special to also be able to describe gravity and its effect on space and time.
In this interpretation, gravity is no longer a force, but a geometric alteration of space.
The theory can explain things such as black holes, and is also the foundation of the description of the Universe itself in modern cosmology.