A beat consisting of two syllables; an unstressed followed by a stressed.
Dryden's Absalom and Achitophel was written in iambic pentameter; each line consists of 5 iambs. Iambic pentameter with each line ending in rhyme is a common characteristic of the heroic couplet.
Here is the first line of Absalom and Achitophel. The correct way to read it would be with a pattern of stresses that alternates each syllable, and goes low high low high, IE: _/_/_/_/_/.
In pious times, ere priestcraft did begin