Questions related to drivers. Drivers are used to control one property with another. Drivers can also use Python expressions to give even more possibilities, e.g. *#frame/2* will give the current frame divided by two. This tag is not to be used for questions relating to hardware drivers.
Drivers are a way to control values of properties by means of a function, or a mathematical expression.
Effectively, drivers consist of:
A driver configuration that specifies zero, one, or more input values using other properties or object transformation channels, and combines them using a predefined mathematical function or a custom Python expression.
An animation F-Curve that maps the output of the driver configuration to the final value to apply to the driven property.
Driver documentation on the Blender Manual.