In just a few words, this haiku-like poem sketches a small meeting on a morning walk beside a canal. The speaker spots a swan and calls it an "old acquaintance," as if they've crossed paths many times before. The swan, though, seems to be "vexed," or annoyed, at the speaker's arrival.
The humor comes from that gap. The speaker treats the bird like a neighbor, someone you nod to on the street, while the swan appears to take offense at being disturbed. We can't know whether the swan is really irritated or the speaker is only imagining it, and that uncertainty is part of the fun. Swans can be hissy and territorial, so the speaker's reading feels believable.
The mood is light, wry, and affectionate. There's a quiet morning stillness in the setting, but the poem's warmth comes from the speaker's gentle self-mockery, being scolded by a bird and somehow enjoying it. It makes an ordinary walk feel like a relationship with the place and its creatures.