The pitch of a roof is its angle: how flat or how high it sits. Generally speaking, low-pitch roofs tend to accumulate water build-up, which can cause them to wear out faster. A higher pitch will have better drainage, meaning fewer down-the-road issues.
Note that pitch only affects the durability of certain types of roofs. Some roofing systems—such as torch-down roofs—will always have a low pitch. Asphalt roofs should be built on a higher pitch.