The holographic principle asks a question: could everything happening inside a volume of space be described by information written on its surface?
Imagine a 3D room. Instead of describing every object and every event inside it, imagine that all the information needed to describe that room is encoded on its 2D walls. The walls would contain a different description of the same physics, not a picture of what is inside.
In certain theories of gravity, this is more than an analogy. A higher-dimensional “bulk” can have an equivalent description in terms of quantum information on its lower-dimensional boundary.
That idea is called holographic duality, and it has become one of the most powerful clues in the search for a deeper theory of gravity and quantum mechanics.