You can gauge the significance of an alert by comparing the actual performance with the expected performance. Analytics calculates expected performance from previous performance values, and calculates significance from the difference between actual performance and expected performance. As the difference between actual and expected values increases, so does the significance rating.
Major Contributors are the segments that are the likely causes of the alert condition. A custom alert can include up to five segments, and with each segment, the number of sessions and the percentage of change in traffic. You can click Go to report for each segment to view the full report and get some insight into what exactly is driving the change in traffic. For example, you may uncover a blog or news article that has a specific reference to one of your products, or find that there's a misspelling in a link on one of your pages that accounts for an unusually high number of site exits from that page. With this kind of information, you can capitalize on something like a positive product review, or prevent a further loss of traffic by fixing errors on your pages.