This line from Do Not Go Gentle into That Good Night is written in iambic pentameter.
An iamb is a foot (rhythmic unit) of two syllables; the first one being unstressed and the second one stressed. Penta = 5. This means that, in this line, we have 5 sets of iambic feet, which should read like this (lowercase unstressed, capital letters stressed):
wild MEN who CAUGHT and SANG the SUN in FLIGHT.
This pattern is also supported and used throughout the whole poem.
