Iambic Pentameter is a type of poem that uses a set meter to define the number of syllables, and the sequence of stresses that the poem will follow. The rules for iambic pentameter are that each line can only have ten syllables, and these are five pairs of alternating stressed and unstressed syllables. It creates a formula for the rhythm, and rise and fall of words within poetry, usually producing a musical quality.
It is easier to understand this if you read a poem using iambic pentameter, but it will generally sound something like this: ba-BUM / ba-BUM / ba-BUM / ba-BUM / ba-BUM. Each of these sets are known as an iambus, and create a sequence of beats.
Different meters have been used in poetry, but the iambic pentameter seems to be the most well-known and successful. William Shakespeare was famous for his use of Iambic Pentameter, and the majority of his poetry and sonnets follow this meter.