Two lines, and the second gives up halfway. Shelley's "Fragment: A Serpent-Face" is barely a poem, yet it leaves a sharp picture behind. It describes one face and nothing else: "His face was like a snake's--wrinkled and loose / And withered--". We are never told whose face it is, and "His" arrives with no introduction, so we are dropped into the middle of someone's looking. The central idea seems to be that a face can stop being human. Age and ugliness strip away everything warm in it until only a creature's skin is left.
The comparison to a snake is the poem's one big move, and what follows it is surprising. The three adjectives, "wrinkled," "loose," "withered," do not describe a snake's usual menace, such as scales, fangs or speed. They describe skin that no longer fits its owner. Snakes shed their skin, so slackness is a believable thing to notice. A snake also carries a heavy cultural reputation for danger and deceit, and the poem never mentions either. Judging by these words, the feeling is closer to revulsion, perhaps with a little pity, than to fear. "Withered" widens the picture again. It is a word for leaves and flowers, not reptiles, so the face seems to belong to two kinds of life at once, animal and dried-up plant, with nothing living and human in between.
The sound of the first line backs this up. It has ten syllables and starts out like the iambic pentameter that English poets treat as their standard line: "His FACE was LIKE a SNAKE's." Then the rhythm stumbles. "Snake's" and "wrinkled" both want a strong stress, so the beat collides, and the dash that follows makes a pause, as if the speaker stopped to look harder. The consonants help too. The line is full of soft hisses (*his, face, was, snake's, loose*), so the sentence about a snake quietly sounds like one. The long *oo* of "loose" ends the line on a slack, open vowel, and the mouth loosens just as the skin does. The repeated *w* in "was," "wrinkled" and "withered" gives a faint, dry whisper.
Rhyme is missing, or at least unanswered. "Loose" is the kind of word that sits at a line-end waiting for a partner, but the second line ends on "withered" and then on a dash, with no echo. The second line is short, only three syllables, and its opening "And" promises a longer list. We get one more adjective, then silence. That cut-off is the most striking structural fact about the piece. The title only tells us this is a fragment, so we cannot know why it stopped. But as a reader it is hard to ignore that the poem ends on the word "withered" and then withers itself. That may be coincidence, but it is a coincidence the reader feels.
The title does something the text does not. In the lines, the face is only "like a snake's," a comparison that keeps the man and the animal separate. The title compresses this into a hyphenated name, "A Serpent-Face," and the "like" vanishes. The simile has become a label, as though the comparison had hardened into fact. Putting title and text side by side is a good way to see how much a single word like "like" can matter.
As for significance, Shelley is one of the major Romantic poets, and he is usually thought of for large, idealistic, soaring writing. This scrap is concrete, bodily and unflattering. Its worth is that of a sketch: a few strokes show how much an image can do without explanation, plot or moral. It also shows what an unfinished piece can offer. Because the lines break off, the reader supplies the rest, wondering who this man is and what the snake-likeness means for him. The poem's last word is an adjective for something dried out and used up, and the gap after it is part of the effect.