Iambic pentameter is the name given to a line of verse that consists of five iambs (an iamb being one unstressed syllable followed by one stressed, such as "before"). It has been a fundamental building block of poetry in English, used in many poems by many poets from the English Renaissance to the present day.
As with any metre, it is not necessary that every line should be entirely slavish in following the rhythm; in fact, being so could make the poem sound dull. Swapping, dropping or adding stressed and unstressed syllables will lend variety to a line without changing the underlying rhythm.
Poems in iambic pentameter may or may not rhyme. Those that are written in continuous lines of unrhymed iambic pentameter are said to be in blank verse, while rhyming couplets in iambic pentameter may be called "heroic couplets", particularly when each couplet closes a thought or sentence on its second line.