Rhythm is the pattern of stresses in a line of verse. When you speak, you stress some syllables and leave others unstressed. When you string a lot of words together, you start seeing patterns. Rhythm is a natural thing. It's in everything you say and write, even if you don't intend for it to be.
Traditional forms of verse use established rhythmic patterns called meters (meter means "measure" in Greek), and that's what meters are — premeasured patterns of stressed and unstressed syllables.
Much of English poetry is written in lines that string together one or more feet (individual rhythmical units). Feet are the individual building blocks of meter. Here are the most common feet, the rhythms they represent, and an example of that rhythm.
Anapest: duh-duh-DUH, as in but of course!
Dactyl: DUH-duh-duh, as in honestly
Iamb: duh-DUH, as in collapse
Trochee: DUH-duh, as in pizza
To build a line of verse, poets can string together repetitions of one of these feet. Such repetitions are named as follows:
1 foot: monometer
2 feet: dimeter
3 feet: trimeter
4 feet: tetrameter
5 feet: pentameter
6 feet: hexameter
So the famous iambic pentameter is a string of five iambs, as in Christopher Marlowe's line from Dr. Faustus:
Was this the face that launched a thousand ships
Duh-DUH-duh-DUH-duh-DUH-duh-DUH-duh-DUH
Here you'll notice that there are five unstressed syllables alternating with five stressed — in other words, five duh-DUHs. As you read more poetry, you'll start to recognize feet and meters.