On a flat rubber sheet, free-falling objects move in straight lines.
However, if there is matter (mass), it will warp spacetime like weights on a rubber sheet, pulled tight. The greater the amount and concentration of mass (density), the more that spacetime warps, and the stronger gravity becomes.
Sun’s mass curves spacetime near its surface. The planets orbit because they follow the straightest possible paths allowed by the shape of spacetime around them - curved paths.
If we could shrink the Sun without changing its mass, the curvature of spacetime would become greater near its surface, as would the strength of gravity. This is equivalent to shrinking the Sun to have the same density as a white dwarf.
If we somehow continued to shrink the Sun, it would eventually make the curvature so great that it would be like a bottomless pit in spacetime: a black hole.
A black hole creates an infinitely deep hole in the fabric of space-time.
Spacetime is so curved near a black hole that nothing can escape it.
The “point of no return” is called the event horizon . Since stars are spherical, a black hole’s event horizon is too and it is a three-dimensional surface.
While the rubber sheet analogy works really well for imagining an extra dimension to the view of the world that we usually have, there are some limitations to this analogy.