To make a dart hit the bulsseye
The Laser-Aimer was a small device invented by Tracy the Gorilla that could be attached to a dart or an arrow to ensure it hits the bulseye every time. Tracy used the Aimer to beat Eddie Spenser Jr. at a game of darts.
When Futura entered a crossbow shooting contest as part of the three tasks demanded by the Sleeping Dragon, Tracy proposed she would attach the Laser-Aimer to the bow. But Futura declined, because she felt she would be cheating that way.
The Laser-Aimer was first used in the episode The Sleeping Dragon.