Suppose we want to get an idea of how many fish live in a pond. Is there an easier way to do this than catching all of the fish in the pond?
Suppose the fish have certain characteristics that are readily discernible, for example, their species. If we caught 100 fish from sufficiently random locations in the pond, we could assume that the sample of caught fish is representative of the population of all fish in the pond. That is, the characteristics of the population should show up in the sample in roughly the same proportion.