The Sargan test is a statistical test used in econometrics to assess the validity of instrumental variables in regression models. It checks whether the instruments used are uncorrelated with the error term, which is crucial for ensuring that the estimates are unbiased and consistent. In practice, the Sargan test evaluates the over-identifying restrictions in a model, meaning it tests whether the additional instruments are valid. A significant result indicates that at least one instrument is problematic, while a non-significant result suggests that the instruments are appropriate for the model being analyzed.