The UI Slider object allows the user to drag a handle along a path. The position on the path corresponds to a range of values.
To create a UI Slider, select Create | UI | Slider. By default, a UI Slider has three children: a Background, a Fill Area, and a Handle Slide Area. The Fill Area also has a child, Fill, and the Handle Slide Area has a child, Handle.
The Background child is a UI Image that represents the full area that the Slider's Handle can traverse. In the default Slider example, this is the darker gray background area that gets filled.
The Fill Area child is an empty GameObject. Its main purpose is to ensure that its child, the Fill, is correctly aligned. The Fill is a UI Image that stretches across the Fill Area based on the Slider's value. In the default Slider example, this is the light gray area that trails behind the handle and fills in the...