Here is how I can explain spacetime. OK, here is the twins paradox. One twin on earth, one twin goes on spaceship and speeds away at a speed near the speed of light,
c. Both are the same age. When the twin comes back, his twin is older than he is because his time was "slowing down".
You can graph his journey on a
space-time graph.
A path through a space-time graph is called a world line. Nothing can pass the light world line, because you would have to go faster than
c. The stationary twins world line does not go anywhere along the x axis because he isn't moving anywhere. The speeding twin's line is going at a small angle to the light world line because he is going at a speed NEAR
c away from his stationary twin. It curves back because he turns around.
This is almost impossible for me to explain in a forum, you just have to research this stuff more. Use books instead of the internet, they are way better.