an event horizon
Imagine a boundary in space that is a perfect one-way door. Cross it heading inward and you can never come back, nor can any signal, any flash of light, any cry for help, ever climb back out to tell the outside what became of you. That surface is an event horizon, the defining feature of a black hole. It is not a wall or a surface made of anything; it is a place in empty space beyond which the future leads only inward.
For a non-rotating black hole the horizon sits at the Schwarzschild radius, r = r_s = 2 G M / c^2. In geometric terms it is a null surface: it is traced out by light rays that, at that exact radius, hover forever, neither escaping nor falling in. Inside it, the roles of the r and t coordinates effectively swap, so that moving toward smaller r becomes as inevitable as moving forward in time is for us, which is the precise sense in which the central singularity becomes unavoidable once you are within.
The most important and counterintuitive point is that the horizon is not locally special. An astronaut falling in feels nothing unusual as they cross it, in exact keeping with the equivalence principle, since locally the spacetime there is smooth and unremarkable. It is a distant observer who sees something strange: infalling light is redshifted without limit and the faller appears to slow and freeze at the horizon, their image fading forever rather than actually stopping. The horizon is a global feature of the spacetime, defined by where signals can ultimately reach, not a local landmark you could feel.
If you watched a friend fall toward a black hole, their clock would appear to tick ever slower and their light ever redder, so you would never actually see them cross the horizon; they, meanwhile, would sail through it in a finite and unremarkable amount of their own time.
Two viewpoints: the faller crosses smoothly; the distant watcher sees a frozen, reddening image.
Nothing physically dramatic marks the horizon of a large black hole; tidal forces there can be gentle. The freezing and infinite redshift are effects seen by outside observers, not experiences of the one who falls in. Horizon-crossing is a global fact about the spacetime, not a locally detectable event.