To sit out means to choose not to participate in an activity, or to be kept from participating. When you sit out a game of dodgeball during gym class, you're on the sidelines watching instead of playing. Maybe you're injured, maybe you need a break, or maybe the teams are uneven and someone has to wait for the next round.

In sports, a coach might decide to have a star player sit out for part of the game to let them rest, or as a consequence for breaking team rules. The player wants to compete but has to watch from the bench instead.

You can sit out other things too. If your friends are planning something you don't want to do, you might say, “I'll sit this one out.” During a dance at a party, you might sit out a song because you don't like it or you're tired.

The phrase suggests a temporary absence: you're not quitting forever, just skipping this particular round or event. There's often a sense that you'll be back soon: the basketball player sits out the third quarter but returns for the fourth, or you sit out one dance but join the next one.