Paceline refers to a group of cyclists who rotate in the lead. The leader advances to the front to meet the wind and then returns to recover.
This riding assists the group in riding more efficiently and conserving energy by trading turns in the front. Everyone pulls in the front for a duration of time, then when finished switches to the end. They must have good comms, consistent speed, and fluid transitions. Pacelines are useful in group time trials, group rides, or working together in road races and crits.