A slider is a control for choosing a value from a set range by moving a handle along a line. It is like moving a volume control from quiet to loud. The ends show the lowest and highest values, and the handle shows the current choice.
For example, a photo tool might use a slider to adjust brightness from low to high. Showing the current value can help when someone needs a precise setting instead of a rough guess.
Sliders work best when the range is easy to understand and a small adjustment is acceptable. If the exact number matters, also show the value or provide a field where it can be typed.