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.