When you configure Evaluation Periods and Datapoints to Alarm as different values, you’re setting an “M out of N” alarm.Datapoints to Alarm is (“M”) and Evaluation Periods is (“N”). The evaluation interval is the number of data points multiplied by the period. For example, if you configure 4 out of 5 data points with a period of 1 minute, the evaluation interval is 5 minutes.

https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/AlarmThatSendsEmail.html