Iambic Pentameter? Sonnets? I need help?

I have to write wedding vows for Romeo and Juliet in the form of a Shakespearean Sonnet. I know it has to be 14 lines with the rhyme scheme ABABCDCDEFEFGG but what I don't get is Iambic Pentameter. It says on my paper "The poem should be written also in iambic pentameter (10 syllables per line, broken into 5 parts)." What exactly does that mean? I'm so confused.