The speaker catches sight of someone, maybe only for a second, and is stopped by what they see: sunlight falling across that person's face. In that brief moment they realize something they hadn't expected. They never thought a plain, passing sight could matter this much, yet here it is, standing in for every poem they might ever read or write.
The feeling is quiet, tender admiration. There's surprise in it, and a little awe, but nothing loud or dramatic. The short lines, which break right after small phrases like "the sunlight" and "the only," slow the poem down so each moment lands separately, the way a glance seems to stretch when you're really looking. The mood is hushed and intimate, like a private thought the speaker is half whispering to themselves. The poem doesn't say who "you" is, which keeps the focus on the feeling: that real beauty can be found in an ordinary moment, with no need for fancy words.