Cost allocation tagging is to label your S3 buckets with a tag that consists of a key and a value in order to track their costs. Of course, cost allocation tagging is used also for other AWS resources such as EC2 instances, EBS volumes, RDS, and so on.
You need to enable Detailed Billing reports and set up your monthly cost allocation report by following the official instructions in advance:
Understand Your Usage with Detailed Billing Reports: http://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/detailed-billing-reports.html
Setting Up Your Monthly Cost Allocation Reports: http://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/configurecostallocreport.html