When conducting a study, a researcher selects a relatively small group of participants (a sample) from an entire population of all possible participants (for example, selecting college students at a couple of colleges from all college students in the world). Ideally, the researcher would have a Representative Sample, which is when your participants closely match the characteristics of the population, which helps you generalize your results from your small group of people to large groups of people.
For example, imagine you are at the supermarket picking out grapes. There are red, green, small, large, and globe grapes. In a representative sample you would have an equivalent number of each type of grape. You could then taste them all and make generalizations about all grapes just from tasting these few because your sample
represents the larger population.