Each of the three short stanzas names a feeling, compares it to something, and then says it was mistaken for something else. Pain is like rust, but "you" took it for salt. Joy is like oil, mistaken for molasses. Love is like fire, mistaken for candlelight. The title, "Before the fall," suggests these misreadings come before some kind of collapse, though the poem never says what that is.
The speaker seems to be looking back at someone, maybe a past self, who couldn't see what was really happening. Rust slowly eats away at things, yet it felt as intense as pleasure. Oil can look sweet and soothe, but it isn't food. The last stanza stings most: a fire that felt small enough to "blow out" was never that gentle.
The mood is quiet, regretful and a little warning-like. Nothing is shouted. The speaker simply lays out how easy it is to misjudge what's dangerous when it feels good.