An iamb consists of an unstressed syllable followed by a stressed syllable.
Shakespeare uses iambic pentameter in most of his poetry and plays because it is the meter most similar to everyday speech. This made it easier for performers to memorize and perform. An iamb is only two syllables but Shakespeare's lines are made of 10 syllables. It's called iambic pentameter because two syllables (one unstressed and one stressed) five (penta) times equals 10 for the meter. Meter in poetry is the rhythm of the line or poem.