I think it means the max amount of information in the observable universe is bounded by the amount of information that could fit on the 2D surface of a spherical event horizon of a black hole of the same size.
That was the origin of the theory, but from what I understand it's been developed beyond that point.
Is there a why? I got curious.
If you take all the mass and energy of the observable universe and throw it into a black hole, its event horizon would expand and encode all the information you put into it. Information can't be destroyed so that's why it is encoded in the event horizon. The diameter of the event horizon would also be much smaller than the diameter of the observable universe, because there's a lot of empty space in the universe, and we don't see everything collapsing inwards due to too much mass. So if you had an event horizon with the diameter of the observable universe, it would encode a lot more information than what's in our observable universe. Note I'm just a layman here and I could be interpreting it incorrectly.