The metaphor of horizon is one that shows up throughout the Heidegger corpus, including as a means of understanding beyng. Here is the last paragraph of
Being and Time. The bold in the last question is mine.
Something like 'Being' has been disclosed in the understanding-of-Being that belongs to existent Dasein as a way in which it understands. Being has been disclosed in a preliminary way, though non-conceptually; and this makes it possible for Dasein as existent Being-in-the-world to comport itself toward entities--toward those which it encounters within-the-world as well as towards itself as existent. How is the disclosive understanding of Being at all possible for Dasein? Can this question be answered by going back to the primordial constitution-of-Being of that Dasein by which Being is understood? The existential-ontological constitution of Dasein's totality is grounded in temporality. Hence the ecstatical projection of Being must be made possible by some primordial way in which ecstatical temporality temporalizes. How is this mode of the temporalizing of temporality to be Interpreted? Is there a way which leads from primordial time to the meaning of Being? Does time itself manifest itself as the horizon of Being?
P. 488
Several aspects of this notion of the horizon as an ontological metaphor work well. The familiar notion of a horizon is that of a ship is at sea, surrounded by a horizon.
When an island is spotted, the ship approaches it, and as it gets closer, the island may be discerned with greater detail. Yet, approach one island close enough, and another island will move away, its details lost until it is merely just another island. So it is with anything. We can examine something more closely and understand that phenomena in more detail, or instead examine some other thing, losing our focus of the first thing.
The ship can also approach the horizon, but unlike approaching a thing like an island, the horizon never gets any closer. As such, the horizon is like beyng. All beings are enclosed by it, but beyng cannot be studied like a thing, it always eludes us when we approach it. Such is the ontological difference between beings and beyng.
Phenomenology can be used to study phenomena, but it cannot directly study beyng, although that limitation does give us some insights about beyng.
Dasein is the ship bobbing along on the ocean, being-in-the-world. It can only appropriate and understand what is within its horizon, the clearing where it encounters phenomena.
Time as temporality is a horizon. We always approach the future and the past recedes from us, like moving toward and away from the horizon at the same time.
The horizon can be nearer or farther away, so that the world encompassed shrinks or grows. From the crow's nest you can see more islands than from the deck; the higher you look from the further you can see. You can observe more phenomena, but not get any closer to the horizon. Instead, the more you know, the further away the horizon is. And there is an absolute limit, the most you can ever see of a sphere is half, and horizon continues to elude you whatever your position.