A Two-Dimensional Analogy
A two-dimensional analogy of the discussion in the left panel
is that no matter which direction you go on the
surface of the Earth in a "straight line" (a
"geodesic" or a "great circle"), you never escape the Earth but instead return
eventually to the same point. Imagine extending that analogy to the four dimensions of
spacetime (with a more complex geometry involving a singularity)
and you have a rough explanation for why light travels at light
speed, but cannot escape the interior of a black hole.
Of course, it is much harder
to visualize the spacetime example in four dimensions. But the idea is the
same and what we can't see with our eyes we can still visualize mathematically.