You manipulate the IV in an experiment in most cases. Sometimes you have to select subjects with appropriate characteristics.

You do this because you can't manipulate the factor (e.g., gender).

You hope the manipulation of the IV will affect the DV. The DV is the variable you measure.

You have to watch out when you manipulate the IV for confounds and the ranges of IV values you choose.

It's really an easy idea.

Change the IV - Measure the DV