Since researchers can't study every person in the world that is of interest to them, they need to study a subset of this entire population, also known as a sample. Then, people are picked from this sample "at random" to participate in the study. It is hoped that the random sample will be representative of the entire population. Often researchers use random numbers table to help them pick participants at random (take a look in the back of your introductory psychology textbook. I bet it has a random numbers table).