What is Line flying under supervision?

LIFUS - Line flying under supervision

Before being released into unsupervised operation, each pilot must undergo on-job training in commercial air transport. In this case the on-job training is a number of route segments to fly under the supervision of a line training instructor This training is known as Line Flying Under Supervision - LIFUS. However, it is often referred to as line training or supervision flying. Supervised operational experience is the term commonly used for the LIFUS under FAA. However, it is always the same on-job training to get the en-route experience in particular aircraft type and variant.