Then a mile of warm sea-scented beach; Three fields to cross till a farm appears;
The speaker has disembarked from his boat, and must now walk a mile on the beach, and then across three fields.
The line "a mile of warm sea-scented beach" is kind of strange; the speaker never says he must cross it, but line 8 implies that that is exactly what he is doing.
"A mile of sea-scented beach" seems to be a dangling or static piece of scenery, as if it weren't doing anything, just like the first four lines of the poem.
Its not clear if the farm is the speaker's destination or not, but it seems likely that it is.
Notice how the farm "appears" after a somewhat strenuous journey (across the sea, across the beach, across three fields). It almost seems magical.
A tap at the pane, the quick sharp scratch And blue spurt of a lighted match,
Apparently the farm is the speaker's destination because now someone (most likely the speaker – why would the person inside the house do the tapping?) is "tapping" at the windowpane.
Someone appears to respond to the tap by lighting a match (the "quick sharp scratch" refers to the sound of lighting a match).
Notice that these lines, like a number of others in the poem, possess a certain static quality; instead of saying "there was a tap at the pane," or something to that effect, the speaker simply says "a tap at the pane."
And a voice less loud, thro' its joys and fears, Than the two hearts beating each to each.
After the "tap at the pane" and the lighting of the match, a voice speaks. Based on what we know of such "meetings at night" (from fairy tales and the like), it seems possible that the voice is that of the woman in the house.
The voice is less loud than the hearts of the two lovers. That sounds kind of strange (we don't usually think of heartbeats as something so loud we can hear), but it suggests that the two lovers are so excited that their hearts beat louder than a human voice.