Heidegger's explanation of what a horizon is, in general.

In Greek, what limits is called to horidzon. A horizon belongs to the essence of living beings in their vitality, to the securing of stability in the form if the need for a schema. Accordingly, the schema is not a limit imposed on the living being from without, not a limit with which life-activity collides so as to stunt its growth.

Forming horizons belongs to the inner essence of living beings themselves. Initially, horizon simply means setting limits to the unfolding occurrence of life with a view to stabilizing the onrushing and oppressing torrent. The vitality of a living being does not cease with this limiting scope, but constantly takes it start from it. The schemata takes over the elaboration of the horizon.

P. 86

Furthermore,

The horizon, the scope of the constant that surrounds man, is not a wall that cuts man off; rather, the horizon is translucent. It points as such to that what has not been fixed, what becomes and can become, the possible. The horizon pertaining to the essence of living beings is not only translucent, it is somehow always measured and "seen through," in a broad sense of "seeing and looking." As an occurrence of life, praxis moves in such seeing-through, in "perspectives." The horizon always stands within a perspective, a seeing-through to something possible that can arise out of what becomes, and only out of it, hence out of chaos. The perspective is a way of looking through, cleared in advance, in which a horizon is formed. The character of looking through and looking ahead, together with the formation of a horizon, belongs to the essence of life.

P. 87

The horizon is not the boundary or limit of what can be seen, but from where new things arise.  ¶ 8:01 AM