The simulation shows the classic physics situation of a monkey and hunter.
In this case, the hunter is trying to tranquilize the monkey, so the monkey can be re-located to a better habitat. The monkey is clever - when the tranquilizer dart (in blue) leaves the gun, the monkey (in purple) lets go of the tree branch, and starts to fall straight down. Note that there is a net (not shown) at the bottom to catch the monkey, so the monkey won't get hurt.
How should the gun be aimed so the dart hits the monkey?

You can explore various parameters, including changing the direction the gun is aimed, changing the value of the acceleration due to gravity, and adjusting the dart's launch speed.