In just four lines, the speaker listens to rain and feels how little they matter next to it. The opening image treats rain as if it had a grammar, a set of rules and patterns that make it speak. That speech is compared to an open road: wide, continuous, going on without needing anyone. Both are "full things," complete in themselves.
The last line turns the focus inward. The speaker says they are "nothing to" this voice, which can mean the rain doesn't notice them, or that they have no place in what it says. The title adds a twist. "Active voice" is a grammar term, and here the rain is the one acting while the speaker is only on the receiving end.
The mood is quiet and humble, a little lonely but not bitter. It feels like standing still while something much larger carries on, and accepting that.