This statement derives from the general relativity theory published by Albert Einstein in
1915. It refers about the event in which an accelerated body crosses the spacetime boundary of a black hole known as the "
event horizon". Beyond this boundary or "
point of no return" the gravitational pull becomes so great as to make escape impossible and thus "you'll never go back".
"If a black hole is an
oyster then
the singularity is
the pearl inside. The gravity is so strong it’s always hidden in darkness, beyond the horizon. That’s why we call it a black hole, once you go black you'll never go back."