In just three short lines, the poem catches a change that happens when daylight fades. Sundown "qualifies" speech, and the word does two jobs at once. It can mean that the fading light limits or tempers what we say, and it can mean that it describes speech, labels it, as "mere excitement." Either way, ordinary talk suddenly looks like noise, a kind of restless energy, set against something quieter and larger.
The speaker doesn't announce a feeling outright, but the tone suggests a hushed, slightly humbled attention. The word "mere" carries a gentle deflation, as if the evening makes chatter seem unnecessary. The poem has no punctuation and very few words, so the white space around the lines does a lot of the work. It leaves a pause, almost a silence, which fits the subject. The mood is calm, reflective and a little mysterious, inviting you to slow down and notice how the end of the day changes the way words feel.