Metrics are the fundamental unit that represents the time-ordered set of data points in AWS CloudWatch. Consider metric as a variable that monitors and the data points as the values that represent the values of different time intervals. For example, the CPU utilization, disk usage, and the data transfer of an EC2 instance are examples of metrics. These metrics expire every 15 months if not used anymore.
I recommend this AWS Migration Certification course by Intellipaat if you want to learn AWS data migration.
Also, watch this video on Amazon CloudWatch: