Three short lines, and a whole scene opens up. It's the first rain of the season, and the speaker stands by a black river where "neon fireflies" are glowing. The phrase is a surprise, because neon belongs to city signs and electric light, while fireflies belong to nature. Putting them together makes the insects look almost artificial, or makes the lights look alive. It's hard to tell which, and that blur is part of the charm.
The speaker doesn't say how they feel, but the poem suggests quiet wonder. "First rains" hints at relief or renewal, as if something has just woken up. The sharp, bright glow against the dark water makes a striking contrast, and the trailing ellipsis at the end lets the moment hang in the air. The mood is hushed, a little mysterious, and deeply observant. It reads like someone pausing to take in a small, beautiful sight before it disappears.