Sideline has two common meanings:

1. The boundary line along the side of a playing field or court. In sports like basketball, soccer, or football, the sidelines mark where the playing area ends. Players who aren't currently in the game wait on the sideline, along with coaches and substitutes. When a ball goes out of bounds by crossing the sideline, play stops. Parents and fans often watch from the sidelines, cheering but not participating in the action.
2. An activity or job someone does in addition to their main work. A teacher might have a sideline business making pottery on weekends, or a student might have a sideline job fixing bicycles for neighbors. These extra activities often start small but can grow into something bigger.

The word also appears in the phrase sidelined, which means being forced out of action. An injured player gets sidelined until they recover. You might feel sidelined during a group project if nobody listens to your ideas, leaving you watching from the metaphorical sidelines instead of actively participating.