A man walks around at the end of the day, with no reason to hurry home. The title, "Flâneur," names the kind of walker he is: someone who strolls and watches for the pleasure of it. The first image is a quiet surprise. The day "lay on a hill," as if daylight were a tired animal settling down while the speaker keeps drifting "here and there."
The second stanza turns from walking to looking. The speaker suggests that looking "ought to be / a continued thing," something that doesn't stop when the light fades. The odd phrase "another supper" can be read as a hint that attention is a kind of nourishment, a meal you can come back to again and again. That is an inference, since the poem doesn't spell it out.
The mood is relaxed, unhurried and gently content. Nothing happens, and that is the point. The speaker simply wants to keep noticing the world a little longer.