A dactyl is a unit of poetry consisting of three syllables, the first of which is stressed. The word "poetry" is itself a great example of a dactyl!
When a poem is made up of three-syllable metrical feet, and the emphasis naturally lands on the first syllable of each foot, you call those units dactyls. The Greek root is daktylos, which means "unit of measure" but also "finger." The literary term came from the "finger" meaning — the three bones in a finger represent the three syllables in a dactyl. These lines from Alfred, Lord Tennyson's "Charge of the Light Brigade" are written in dactyls: "Flashed all their sabres bare, / Flashed as they turned in air / Sabring the gunners there."