A speaker sits outside in the dark and counts raindrops, one by one. That's the whole scene, and the poem's charm comes from how simple it is. The counting appears right on the page, in short lines like "One drop / Two drops / Three drops," and it keeps breaking into the speaker's thoughts, much as counting does when your mind starts to drift.
The speaker seems calm, even content. They say they could stay out all night, and they wonder what else a person could do on a rainy night "other than enjoy its nature." Near the end, they admit they lose count and simply start over, which suggests the number doesn't matter. The counting is really an excuse to slow down and pay attention.
The mood is quiet, patient and a little dreamy. The repetition works like a lullaby, and the closing "All night long" leaves the feeling that the rain, and the speaker's peaceful watching, could go on indefinitely.