Critical Region refers to a set of values that will cause a researcher to conclude that the treatment or intervention she applied has a significant effect on the variable being investigated. Values that fall in the Critical Region signify that the results obtained represent a real difference, and could not have occurred by chance.
When a researcher conducts a study, the data that she gathers needs to be converted into an appropriate statistic, in order for them to be subject to interpretation. For example, if she was using a statistic called the t-test, she would convert the results into a t-value. This t-value will then be compared to a critical value, that is determined by referring to a table that lists down critical values for the t-test. The critical value is the cut-off value that will delineate the Critical Region. Any value higher than the critical value is said to fall in the Critical Region.