An event horizon is the point of no return around a black hole. Once anything, matter, light, or information, crosses it, the black hole’s gravity is strong enough that no path through spacetime leads back out again, no matter how much energy is applied.
It is not a physical surface, there is nothing solid to touch or crash into. It is simply the boundary where the escape velocity needed to get away equals the speed of light, which nothing with mass can reach and which even light itself cannot exceed.
For a non spinning black hole, the size of the event horizon depends only on its mass, and is called the Schwarzschild radius. Everything that happens inside that boundary is permanently cut off from the outside universe, which is part of why black hole interiors remain one of physics’s deepest open questions.
For a large enough black hole, crossing the event horizon would not feel like anything unusual at the moment it happens. There is no local signpost or sudden jolt, an observer falling in could sail past the boundary without noticing anything special, since the horizon is defined by what an outside observer can no longer see rather than by any local physical effect. The real danger comes later, from tidal forces that stretch an object apart as it falls closer to the center, an effect nicknamed spaghettification. For a supermassive black hole, that stretching happens gradually enough that a falling observer could survive crossing the horizon itself, only to be torn apart much deeper inside.
Just outside the event horizon of a non-rotating black hole sits another notable boundary called the photon sphere, a region where gravity is strong enough that light itself can be bent into a circular orbit. It is inherently unstable, any tiny disturbance sends an orbiting photon either spiraling in or escaping outward, but the photon sphere is what produces the bright ring of light visible around the dark silhouette in real images like the one captured by the Event Horizon Telescope.