An iamb is a metrical foot made up of two syllables: the first unstressed, the second stressed. In English verse, it often sounds like beLIEF, deLAY, or reTURN. Poets use iambs to build meter such as iambic pentameter, where the pattern of weak-strong beats helps shape the poem's rhythm.
Example
The wind grew calm beside the gate,
then shook the bare branch back and forth.