A biologist wants to know whether a plant's growth is affected by the material it's planted in. She plants lima bean seeds in a jar of dirt, a jar of cotton, and a jar of rocks, then provides each jar with equal amounts of water, plant food, and sunlight. In this experiment, the independent variable is the material in which the plant is getting planted.
The independent variables are the variable that are in control of the experimenter. The independent variables do not depend on the other variable for their selection. The changes in a dependent variable is studied by varying the independent variable accordingly.