Stealth is the ability to get inside a place unseen, or spy on someone without the person they are spying on realize they are there. This ability is especially useful to characters who have to go up against a large number of armed men. Batman is known for his stealth. He can take out several armed men without them ever even seeing him.
When Marvin and Wendy were at the Gotham City Observatory with the other Justice Leaguers, they saw an oddly dressed man that they recognized from photos from the Justice League Hall of Fame that was taken from the SDI screen. They followed him, attempting to find out where he was going, and they did it using stealth. Nevertheless, the amateur superheroes were unable to stay on him and they lost him in the botanical garden.