Three short lines, and a whole scene opens up: a river in the dark, and the moon above or on it. The surprise comes in the title and the middle line. "Freight" is cargo, something carried, and "the necessary matter of the moon" suggests the river is hauling moonlight downstream like a barge loaded with something that has real substance and real importance.
Notice the word "matter." It can mean physical stuff, but also something that matters. The moon is treated as both. The speaker doesn't say how they feel outright, but the hushed, careful wording suggests quiet wonder, maybe a sense that this ordinary sight is essential in a way that's hard to explain.
The mood is calm, still and a little mysterious. There's no action, just watching. By borrowing a word from shipping and work, the poem makes the night feel less like decoration and more like something that is steadily, patiently moving past you.