From Halopedia, the Halo wiki
Rooftop Runner is an achievement in Halo: The Master Chief Collection. It is unlocked by beating the 15 minute par time on the level Outskirts. The achievement is worth 10 Gamerscore and can be unlocked on any difficulty.
The name of the achievement is a reference to the fact that players must skip large sections of the level by running over across the rooftops in order to beat the par time.