In just four lines, the speaker describes a strange, dreamlike discovery. They find solid ground in an unexpected place: "in back of the sea," as if the land were hiding behind the water. That fits the title, since marshlands are neither fully land nor fully sea, and the poem seems to live in that blurry in-between space.
The phrase "mouth to mouth" is the most startling. It could suggest the meeting of water and shore, or something as intimate as a kiss or a shared breath. Then the speaker is "hunting a moth of fire," an image that joins delicate and dangerous things. A moth is drawn to flame, but here the moth itself is made of fire, so the speaker is chasing something glowing and fragile that may be impossible to catch.
The mood is quiet, curious, and a little magical. The speaker seems driven by wonder and longing rather than fear, following a flicker through a half-real landscape.