All experiments examine some kind of variable(s). A variable is not only something that we measure, but also something that we can manipulate and something we can control for.

An independent variable, sometimes called an experimental or predictor variable, is a variable that is being manipulated in an experiment in order to observe the effect on a dependent variable, sometimes called an outcome variable. The dependent variable is simply a variable that is dependent on an independent variable(s).

Example: A tutor wants to know why some students perform better than others in a Maths test. She thinks that it might be because of two reasons: (1) some students spend more time revising for their test; and (2) some students are naturally more intelligent than others. She decides to investigate the effect of revision time and intelligence on the test performance of the 100 students. The variables for the study would be:
• Dependent Variable: Test Marks (from 0 to 100)
• Independent Variables: Revision time (in hours) Intelligence (IQ score)

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Sources:
https://statistics.laerd.com/statistical-guides/types-of-variable.php