Convenience Sampling is a sampling method (a way of gathering participants for a study) used where a you select a naturally-occurring group of people within the population you want to study.
Let's say you are a marketing student who works part-time as a gym instructor. Your class project is to conduct market research to find out whether people preferred their energy drink in a can or in a plastic bottle. You take your gym class and have them try out both kinds of packaging to determine which one they liked better. Your gym class is but a small portion of the entire energy-drinking population, but you select them for your study because it is convenient for you to do so.