Generally, either with little to no meaning or just repeating what's already been said. These are common in oral traditions (a lot of Homeric epithets can be considered chevilles) but by no means restricted to them. Adopted in the 19th century from French, where it's literally a peg or plug, with the latter sense being operative; that in turn is from Latin clāvīcula, key/pivot/peg, from a Proto-Indoeuropean root.
You can comment here or there.
"When writing poems, if your meter's not tight,
Just add a cheville, and it all adds up right."
Or a less ad hoc example, from Shakespeare's sonnet 38:
"How can my Muse want subject to invent,
While thou dost breathe, that pour'st into my verse"
A lot of do/doth/dost auxiliaries in verse are not needed even for the emphasis they give the verb, and are for the meter.
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