Iambic pentameter is meter that uses five iambs, or a no-stress/stress grouping of syllables, to comprise one line of poetry.
An example of iambic pentameter follows. Stressed syllables are capitalized; unstressed syllables are not. A fairly common example of iambic pentameter is:
|“||to BE or NOT to BE (caesura) THAT is the QUEStion||”|
You will note that the example is not perfect. Iambic pentameter need not always be precisely formal. In fact it rarely is. Notice that in this quote from Matthew Arnold's "Dover Beach," the meter is not precise, but the opening and ending feet are distinctly in iambic pentameter, and the line is otherwise predominantly this style:
|“||and WE are HERE as on a DARKling PLAIN||”|