When conducting an experiment, what is provided by the independent variable?

The independent variable provides the control required to obtain output measurements or results.

Explanation:
The purpose of an experiment is to determine the response of a dependent variable to changes in an input variable.

The input variable controls the output measurement, and it is, therefore, the independent variable.

Example:
I place a mass of water in a beaker and insert an electric heating coil into the water. I place a thermometer in the water to measure the average temperature of the water as I increase the heat input to the water by varying the current to the heating coil.
Because I am controlling the output result (temperature) by varying the input (current), my independent variable is current.

As a function,
T = T(I)
where
I = current (the independent variable),
T = temperture, (the dependent variable).