The horizon of being is also the horizon of intelligibility or meaning. Heidegger says at the beginning of Division II of Being and Time.

What we are seeking is the answer to the question about the meaning of Being in general, and, prior to that, the possibility of working out in a radical manner this basic question of all ontology. But to lay bare the horizon within which something like Being in general becomes intelligible, is tantamount to clarifying the possibility of having any understanding of Being at all--an understanding which itself belongs to the constitution of the entity called Dasein. The understanding of Being, however, cannot be radically clarified as an essential element in Dasein's Being, unless the entity to whose Being it belongs, has been Interpreted primordially in itself with regard to its Being.

P. 231

He will interpret temporality in Division II to show that time is critical to understanding the horizon of beyng.  ¶ 6:44 AM