Four lines, and the speaker has confessed, defended himself and changed the subject. "I rant" is as blunt as an admission can be: two words, a full stop, no excuses. The joke is that a poem about ranting is itself tiny and tightly controlled. Whatever the speaker's mouth does when it runs away with him, the poem does not. That gap between the wildness it names and the discipline of its form drives the whole thing.
The second sentence moves from confession to puzzled self-observation. "My mouth must have / some good Latin to flaunt" treats the speaker's own mouth almost like a stranger in the house. He doesn't say "I have Latin." He deduces that his mouth must have some, the way you might guess a neighbour owns a piano after hearing it through the wall. The word "must" suggests a rant is not something he plans. It pours out, and he works out afterwards what was in it. The line break after "have" lets the verb hang for a moment, so the reader briefly wonders what the mouth has, until "some good Latin" arrives. "Latin" suggests the language of church, law and scholarship, so these rants sound learned, even authoritative. "Flaunt" is a sharp, slightly guilty verb. It admits vanity, as if the speaker suspects he enjoys the performance. "Good" is nicely double-edged too, meaning either fine, impressive Latin or Latin that's simply plentiful.
Then the poem turns on its simile: "like a monk in the palace / coming to look after his mind." A monk is the opposite of a ranter, a figure of silence and restraint, and a palace is the opposite of a cell, a place of display and power. Putting one inside the other suggests someone out of place, carrying old, quiet learning into a room built for show. The last line is open to more than one reading. "His mind" most naturally belongs to the monk, so he has come to the palace to tend his own mind, perhaps to protect it from the noise around him. But the speaker is comparing himself to the monk, so the mind in question is also his. On this reading the rant is not a loss of control but a form of upkeep, a strange kind of care. The "Latin" he flaunts becomes the monk's old discipline, still working inside the commotion. There is also a faint sound-echo that readers may or may not hear: "palace" sits one letter from "palate," the roof of the mouth, which is where this poem begins.
The sound of the poem supports this. The first line is nearly a regular beat, "I RANT. My MOUTH must HAVE", which marches like someone warming up. After that the rhythm loosens, and there is no rhyme scheme to hold things in place. The only gesture toward rhyme is "rant" in the first line and "flaunt" at the end of the second, a near-echo that pairs the act with its showiness. The lines also grow slightly longer, at about six, six, seven and eight syllables, so the poem seems to gather speed in the way a rant does. Meanwhile the letter m moves through the lines: "My mouth must", "monk", "mind". The mouth-sound is almost chewed into the vocabulary. The poem's path from "mouth" in line one to "mind" in line four is its real movement, from the noisy body to the thinking part that needed looking after all along.
The title is worth a moment too. "Ranter" turns the verb of the first line into a label, a kind of person, and the speaker seems both to accept the label and to wriggle out of it. In English history "Ranters" was also the name of a radical religious group in the seventeenth century. I can't say whether the poet had them in mind, but the monk and the Latin do give the poem a religious backdrop, which makes that echo possible.
What makes the poem worth reading is how much self-awareness it fits into so little space. It is funny and unembarrassed, and it avoids both pride and apology. It suggests that an outburst, the thing we are usually told to be ashamed of, might be the mind's way of staying in working order. A short poem that quietly overturns what we assume about a loud habit is a small achievement, and this one manages it in a single sentence.