Selection bias occurs when the participants included in a study or survey are not representative of the larger population. This can lead to skewed results and conclusions that do not accurately reflect the true situation. For example, if a researcher only surveys people who visit a specific gym, the findings may not apply to the general population's fitness habits. This bias can arise from various factors, such as how participants are chosen or who volunteers for a study. It is important for researchers to recognize and address selection bias to ensure their findings are valid and reliable, ultimately contributing to better decision-making in fields like healthcare and social science.