An event horizon is a concept related to black holes, a theoretical prediction of general relativity. It is defined as the boundary surface beyond which no event can affect an external observer. According to the theory of relativity, space and time form a single complex called space-time, which is deformed by the presence of mass or energy.
According to a definition given by Roger Penrose, the event horizon in a black hole is a particular surface of space-time that separates places from which signals can escape from those from which no signals can escape. In a much more general sense, if by event we mean an observable phenomenon, identified by the four space-time coordinates, an event horizon can be defined as a region of space-time beyond which it ceases to be possible to observe the phenomenon.
In the case of Schwarzschild black holes, the event horizon is a spherical surface surrounding a singularity at the center of the sphere; the latter is a point at which the density would be infinite and the laws of physics, according to the theory of general relativity, lose their meaning.
The singularity may not be necessary, according to some theories of quantum gravity, which postulate space-time as an entity with a physical reality, and not just a mere mathematical concept, divided into discrete elements with a diameter of a Planck length.
In other words, space-time would have, according to the aforementioned theory, a physically active, not passive role and its intimate structure would be made up of real atoms which would form a dense network in constant evolution.
Under normal conditions, the atomic structure of space-time would not be perceived, which would appear to be a mathematical continuum and the Universe would be described by general relativity, but at distances in the order of the Planck length, things would change radically: the quantum and gravitational effects would assume comparable intensities.
It would be as if the space assumed its own physical personality and interacted with energy in an active way. A very common mistake is to imagine the event horizon of a black hole as a more or less spherical static surface.
What is better to keep in mind, however, is that it is a horizon to all intents and purposes, or rather something that cannot be reached and which moves away as an observer approaches.