On the surface, this is almost nothing: someone says they won't be walking the dog tonight. It's an ordinary chore, mentioned in three short lines. But the poem works by making us wonder why the speaker says it at all.
The spareness is the point. Because we're given no reason, we fill the gap ourselves. Maybe the speaker is tired, ill, grieving, or simply overwhelmed. Maybe the dog is no longer there, and the sentence is a quiet way of facing an absence. The poem never says, so any of these readings stays open. The line breaks add to the effect. Each pause makes the statement feel slower and heavier, as if the speaker is pushing the words out one at a time.
The mood is flat, weary and a little lonely. The speaker doesn't seem angry or dramatic, just worn down or withdrawn. Part of the feeling comes from how small the thing is: when a daily habit gets dropped, it can say more than a big confession would.