1. In sports, to stay outside the boundaries of the playing area because one is not participating in the game, due to being a substitute, injured, etc. Unfortunately that knee injury will force her to stay on the sidelines for the rest of the season.

2. By extension, to not be an active participant in something. If you want the boss to notice you, you can't just stay on the sideline during these meetings. Speak up and suggest some ideas! It appears as though the superpowers will be staying on the sidelines, rather than taking an active role in this conflict.