In physics, restitution is defined as "the return of an object or system to its original state after elastic deformation", but as the fixtures in the GameMaker Studio 2 are really rigid bodies and cannot be deformed, restitution is really a way of saying how "bouncy" the fixture is. This setting will affect how much an object "bounces" when it collides with other objects and is co-dependant on other forces that act on the instance like gravity and friction, and is usually a value between 0 and 1 (higher values can be used but may give unpredictable results). Here is an illustration of how it works:
|fixture||The index of the fixture|
|restitution||The restitution of the fixture (usually between 0 and 1)|
The code above will set the restitution of the fixture indexed in "fix_ball" to 0.9.