Scholars talk about Sir Gawain and the Green Knight as part of the "alliterative revival," a literary movement in England in the late 14th century when many poets began writing in a distinctive alliterative style. In this style, a single line of poetry is held together by two words at the beginning of the line that alliterate (share the same first sound), rather than with elements like meter or rhyme. Here's an example:
And all his vesture verayly watz clene verdure,
(And truly all his clothing was brilliant green,)
Bothe the barres of his belt and other blythe stones.
(Both the bars on his belt and other gay gems.)
These lines are each made up of two parts. In the first part, two of the words alliterate, or share the same first sound. Here, they are vesture / verayly (sharing the "v" sound) and barres / belt (sharing a "b" sound). The second part of each line contains one syllable that alliterates with the first two: verdure (to go with vesture and verayly) and blithe (to go with barres and belt). The two parts of the line are separated by a slight pause, called a caesura. Below we've marked the caesura with "//":
And all his vesture verayly // was clene verdure,
Bothe the barres of his belt // and other blythe stones.
All of the alliterative revival poets used this style in their poetry, but the author of Sir Gawain also incorporates a bob-and-wheel at the end of his stanzas. The "bob" is a short connecting line, sometimes only two syllables in length, that connects a four-line ABAB rhyming section in iambic trimeter to the rest of the stanza. Here’s an example:
The fole that he ferkkes on fyn of that ilke,
(The horse that he rides entirely of that color,)
A grene hors gret and thikke,
(A green horse huge and strong,)
A stede ful stif to strayne,
(A proud steed to restrain,)
In brawden brydel quik;
(Spirited under bridle,)
To the gome he watz ful gayn.
(But obedient to the man.)
Here, the "bob" is sertayn. It connects the "wheel" – whose rhyme scheme with thikke (A) / strayne (B) / quik (A) / gayn (B) is ABAB – to the rest of the stanza. The meter of the wheel, three instances of stressed /unstressed syllable pairs, is iambic trimeter (which makes the sound da DUM da DUM da DUM).