A lift hill, or chain hill, is often the inital upward section of track on a typical roller coaster that initially transports the roller coaster train to an elevated point. Upon reaching the top, the train is then disengaged from the lift hill and allowed to coast through the rest of the roller coaster's circuit.
Lift hills usually propel the train to the top of the ride via one of two methods: a chain lift involving a long, continuous chain which trains hook on to and are carried to the top; or a drive tire system in which multiple motorized tires push the train upwards.
External links
CoasterQuest.com - How a roller coaster is propelled