Shakespeare composed much of his plays in the form of poetry, often in a meter called iambic pentameter. Even today, iambic pentameter is the most common meter used in English-language poetry. A regular line of the meter contains roughly ten syllables, with heavier stresses falling on every other syllable.
An iamb is a metrical unit, or a "foot" of meter, made up of one unstressed syllable followed by one stressed syllable ("alive," "forget," "a dog").
Pentameter refers to the number of iambs in the line (penta is the Greek word for five, as in a pentagon). So there are five iambs in a line of iambic pentammeter. Blank verse is unrhymed iambic pentameter.
Here are two examples from Romeo and Juliet. ('bold' means stressed and 'italics' means unstressed)
My grave is like to be my wedding bed.
But soft, what light through yonder window breaks?