The speaker shows off a flock of geese they have cared for, keeping them in the light, letting them fly freely and sleep in a mound. In return, the geese gave a "gift," and the speaker keeps asking whether we can see it, pointing to the geese's beauty and their might. The tone is proud, almost like someone holding up a prized possession and inviting you to admire it.
Then the last line shifts everything. The speaker admits to losing "one thing" and asks us to guess what it is. The poem never answers, so we're left to wonder. The title, "Barter," suggests a trade, which hints that the geese's gift came at a price. Judging by the poem alone, the lost thing might be something like freedom, innocence, or a connection of some kind, but that is an inference, not something the text states.
The mood begins cheerful and boastful, then turns uneasy and wistful, with a quiet sadness underneath the praise.