Answer:
The speaker is at sea at night, heading towards the black land in the distance. He briefly paints a picturesque image of night at sea but moves forward until he pulls his vessel up on to the sand.
He walks a mile along the beach and then across three fields until he approaches his goal, a farm. He taps at the window, sees the lighting of a match, and then is overwhelmed by the beating of his and his lover's hearts as they reunite.
A short and relatively simple love poem, this piece still presents the subtext of the importance of movement in life, and of the dichotomy between the stasis of art and the action of life.
The entire poem has a sense of movement to it that reflects the speaker's desire to reunite with his love. The poem's meter and sound clearly denote a sense of pressing intent. Read it aloud to sense how the language is pushing ever forward, with three lines in the first stanza alone beginning with "And," as though to suggest that what is on the speaker's mind is never the moment he is in but rather the next thing, since the latter gets him closer to his lover. Technically, the meter is iambic tetrameter, though it is hardly strict, as should be expected in a poem that puts movement over order and contemplation.
This sense of movement is particularly interesting when compared to what is usually expected of a poem of this sort. The imagery, especially in the first stanza, is extremely picturesque and pastoral, the type of landscape that readers often expect poets to spend time contemplating and describing. Poetry, after all, often attempts to capture the complexities and beauty of particular moments, diving deeply into one image to discover all of its profundity.
This speaker, however, is uninterested in the magnificence of "the yellow half-moon large and low." Instead, his focus is on bypassing such elements so as to get to the beach, so he can get to the fields, so he can get to farm. The message here from Browning, who as usual makes no attempt to place himself directly into the work, seems to be that he chooses life rather than art, that for him the goal is movement and energy rather than static contemplation.
But when the speaker arrives to his love the poem abruptly ends. The fact that attainment itself does necessitate a third stanza can imply one of two things: either we can believe that the next action would be further movement of this sort, or we can believe that once he has attained his happiness, he has no further need for writing. He has achieved the unspeakable beauty of love, but as we see in the poem, he as speaker is not interested in plumbing the depths of beauty. Therefore, once he achieves such beauty and happiness for himself, he needs not write but rather can simply live.
It's worth noting the implications of secrecy in the poem. First, the journey and reunion happen at night, suggesting a veil of transgression that in the Victorian age would likely be linked to sexuality. Perhaps there is autobiographical impetus in exploring the theme from this angle, considering that Browning had only recently wed Elizabeth Barrett Browning after a courtship that they had to keep secret from her oppressive father. Many scholars see in it a representation of this courtship, though Browning's general eschewal of autobiography in his poetry makes it hard to imagine he would pursue that so explicitly. Regardless, the sexuality does add a certain sense of danger to the poem. Not only is sexuality implied in the clandestine meeting, but the image of the boat charging into the beach, where it can "quench its speed I' the slushy sand" is easy to interpret as a metaphor along these lines.
Overall, the poem is not subtle in its themes. The speed with which it can be read, since it is only twelve lines long, is the final implication that for he who loves, there is no cause for stopping to admire surrounding beauty, at least not until the supreme beauty of his beloved can be realized.