As I said, General Relativity in all its glory is an impenetrable wall of mathematics such that, if you ever have the misfortune to come across someone who says they understand it, your best bet is to offer a slight smile and back away carefully. Do not let anyone try to lecture you in it, and never, ever, get stuck in a lift with such a person, or you will be subjected to the following.
General Relativity (affectionately known as GR) concerns itself with accelerations and gravity. The basic idea is that the laws of physics in an accelerating frame of reference are the same as those in a gravitational field. If your download speed will let you, have a look at the elevator animation (you’ll see what I mean about lifts), or alternatively you might wish to try the following experiment at home:
Apparatus. Two physics laboratories, a space rocket, two physicists and a baseball bat.
Method. Place one physics lab inside the space rocket. Use the baseball bat to render the two physicists unconscious. Place one in each lab. Now launch the rocket into deep space such that it accelerates away from Earth at an acceleration of 9.8 metres per second squared. When the hapless physicists wake up, their task is to determine whether they in the lab that’s on the earth, or in the lab on the spaceship. (No windows). They are allowed to do whatever experiment they like.
Results. They can’t work out which lab they are in. (Actually, that’s not quite true, because the one on Earth can do experiments to show that the lab they are in is rotating once every 24 hours, but let’s leave that issue aside. We could always spin the rocket as well).
Conclusion. GR.
That’s basically the concept of GR in a nutshell – living in a gravitational field is equivalent to living somewhere that is accelerating. From that, comes a huge range of implications like the bending and redshifting of light close to a massive object like a star, but that’s where you need the maths, and that, believe me, is not suitable material for a blog.