General relativity is a description of gravity first proposed by Albert Einstein in 1915 (building on the work of Sir Isaac Newton which attempted to explain gravity back in the late-1600s). Despite gravity being one of the longest known forces, and the one we are most familiar with in every day life, it is also the least well understood. Newton correctly explained that every particle and object in the Universe has an attractive force which can be felt by every other particle, the strength of this force being proportional to its mass - the more massive the stronger the force. He also found that the further you moved away from an object the weaker the force became (proportional to the distance squared). The descriptions proposed by Newton, known as the Law of Universal Gravitation, works most of the time, but was found not to be wholly accurate - especially in astronomy where we deal with very strong gravitational fields, due to the massive size of objects in the Universe (e.g. stars, planets and black holes).
Einstein's theory said that because objects in space are so massive, they actually cause space (and time) to curve around them (see image 2). The amount of curvature mainly depends on the mass of the object, related to its energy. This allowed for predictions to be made regarding how this curvature could be observed, for example, that light coming from material behind the massive nearby objects would be bent around them, following the curvature. This was proven by Arthur Eddington in 1919 during observations of stars behind the Sun during a solar eclipse.
The theory also implied that black holes would exist, objects so massive that not even light would be able to escape from them, able to cause space to curve significantly, but not be observed themselves. Some of the first strong evidence for their existence came in the 1990s when astronomers began to track stars orbiting around the centre of our Galaxy, the Milky Way, and noted that they appeared to be orbiting around an object which could not be seen. By calculating the speeds and orbits of these stars we were able to estimate the mass of the black hole itself, thought to be over 4 million times the mass of our Sun. A black hole was 'imaged' for the first time in 2019.
The theory also predicted the existence of Gravitational Waves, detected for the first time in 2015.
All of the testable predictions of Einstein's theory of general relativity which have now been conducted agree with the theory and it's currently accepted as being the best explanation of gravity.
Another prediction of the theory is that space and time are linked together - it is the most accurate description of motion, and explains what happens when objects travel close to the speed of light. The famous E=mc2 equation comes from this theory, where E is energy, m is mass and c is the speed of light - it makes energy and mass approximately equivalent. One strange consequence of the theory is that time is relative to the people observing it, if someone is moving quickly then they experience time to move more slowly than someone observing them from elsewhere (moving much slower). The larger gravitational field experienced by the person moving quickly causes time to slow down, but only for them - this is known as time dilation, and the effect is stronger the faster a person/object moves. This has even been tested on the Space Shuttle where very accurate clocks ran more slowly than the same reference clocks on Earth due to the speed the shuttles were travelling! Additionally clocks on satellites orbiting the Earth run more quickly than those on the Earth's surface, as they are in a weaker gravitational field, in fact GPS (used for satellite navigation) only works if we take these different speeds into account!