Each stanza (or "sestet," the name for a stanza with six lines) in the ballad "The Highwayman" follows the same rhyme pattern, which is AABCCB. In other words, the first two lines rhyme ("trees ... seas"), as do the third and sixth ("moor ... door") and the fourth and fifth ("riding ... riding"). This regular rhyme pattern creates a steady, driving rhythm which echoes the fast-paced drama of the poem.

There are also other rhythmic techniques used in the poem. For example, the fourth and fifth lines are always shorter than the other four lines in each stanza. These shorter lines quicken the pace at the same point in each stanza, each time hastening the reader towards the conclusion of the action described in the first three lines.

Finally—and this last technique can be quite difficult to—you could look at the prosody of the poem. Prosody refers to the number of feet in each line. A foot, in poetry terms, is a combination of usually two or three syllables with one or two of those syllables emphasized. A poet will often repeat the same feet over and over again to lull the reader's voice into a pattern or rhythm of emphases. So, for example, "The Highwayman" is mostly made up of feet called iambs (two syllables, where the second is emphasized) and anapests (three syllables, where the third is emphasized). For example:

"The wind /was a torr/ent of dark/ness among /the gus/ty trees.

The moon /was a ghost/ly gall/eon tossed /upon cloud/y seas.

The road /was a rib/bon of moon/light o/ver the purp/le moor"

I assume you are referring to the poem "The Highwayman" by Alfred Noyes. This poem is written in ballad form, with each stanza comprising of six lines. These lines rhyme in an aab cc b structure. This contributes to the poem's sense of melody, as if it were meant to be sung; as a long, narrative poem, it harks back to an earlier age when stories would have been sung in poem form in order to transmit news and sensationalist stories from village to village, often where people were illiterate.

Noyes' poem tells the story of "Bess, the landlord's daughter" and her forbidden love for the Highwayman, who spends his time holding up coaches on the dark roads and stealing from the rich. Ultimately, Bess's dedication to her highwayman leads to her death, as she shoots herself to warn him that he is about to be caught.