Ten syllables to each line. These syllables are broken up into five pairs - this is the pentameter. The 'rhythm' of these syllables is the iambic part. Iambic just means that the first syllable is unstressed whilst the second syllable is stressed therefore the pattern is unstressed, stressed (five times). You may find that a lot of lines start with a stressed syllable (stress, unstress is a trochee) which is used for a stronger effect.