When doing research on a large population, it is often impractical to study every single member of the group. Instead, we look at a portion or sample of the population. Probability sampling refers to methods of selecting individuals to include in a study where each member of the population has an equal chance of being selected.
For example, I am a magazine publisher and I want to conduct a survey that will determine which sections of my publication are most liked or disliked by my subscribers. Let's say I have 1,000 subscribers and I decide to survey 100 of my readers. If I were to use probability sampling, I could assign a number to each subscriber and then use a random number generator to identify which ones I will include in my survey. This way, every reader has an equal chance of being selected to participate.
When done properly, Probability Sampling ensures that the sample has a similar composition and profile as that of the entire population, and that the results you get will be similar if you had surveyed every member of the population.