The Kruskal-Wallis test is a non-parametric statistical method used to determine if there are significant differences between three or more independent groups. It is an extension of the Mann-Whitney U test and is particularly useful when the data does not meet the assumptions of normality required for parametric tests like ANOVA. This test ranks all the data points from all groups together and then compares the average ranks between the groups. If the test shows a significant result, it indicates that at least one group differs from the others, prompting further analysis to identify which groups are different.