Ovid's Metamorphoses, in common with a good deal of Latin poetry, was composed in dactylic hexameter. This kind of meter must have six feet to a line. A foot is a collection of syllables, which can be either long or short. A dactyl is a foot which is composed of three syllables: the first is long, and the next two are short. Indeed, the word "poetry" is itself a dactyl, and consists of one foot. However, in some cases a spondee can be used instead. This is a foot which consists of two long syllables.

Here's an example of dactylic hexameter from the Metamorphoses (3.570-71):

at qua | cumque tra | bes ob | structaque | saxa te | nebant spumeus |

The vertical marks divide the line up into six individual feet. Dactylic hexameter was widely used in ancient poetry, not least because it was meant to be read aloud and this particular metre was more conducive to a public recitation. Dactylic hexameter is inextricably linked to the tradition of epic poetry in both ancient Greece and Rome, most notably in Homer's Odyssey and Iliad and Virgil's Aeneid.