An iamb or iambus is a measure of rhythmic meter in poetry. It's a two beat rhythm using syllables, with the second beat longer, like 'A, AA.' An alternative view of the beat is Weak/Strong, as in 'This Is...' This is a Western poetic rhythm, based on old Greek methods in a form of poetry called 'iambi', although the Greek poems didn't necessarily use the rhythm. The iamb form was preserved through the Roman/Latin culture and passed on to modern European culture. The wider use of the iamb as a measure is in the various rhythmic patterns of poetry, like trimeter, pentameter, heptameter, etc. with the 'iambic' qualifier describing the style.
Examples of Iambic: