When participants sign up for a behavioral experiment, they typically sit at a computer where they receive visual or auditory stimuli and press response buttons according to the instructions they received from the experimenter.
What will be presented on the computer screen or via the headphones varies, but all of these experiments have in common that we record measurements of a person's behavior. Most of the time we tell participants to respond as quickly and accurately as possible. When we analyze their reaction times and errors (because, of course, everybody makes them!), we can draw conclusions about how the brain works.