Critical region and critical values
The critical region is the range of values that results in rejection of the null hypothesis. It can be established using a probability distribution of all potential outcomes. For example the critical region at the 5% significance level is when the combined probabilities of the outcomes comes to 0.05 or just less than that.
The critical value can determine whether or not an outcome is in the critical region. Critical values can be calculated or for random samples tables showing the statistic for each sample size can be used.