VPC Flow logs is a feature allowing the monitoring of traffic flow to and from interfaces within a VPC

VPC Flow logs can be added at a VPC, Subnet or Interface level.

Flow Logs DON'T monitor packet contents ... that requires a packet sniffer.

Flow Logs can be stored on S3 or CloudWatch Logs

https://docs.aws.amazon.com/vpc/latest/userguide/flow-logs-s3.html

https://docs.aws.amazon.com/vpc/latest/userguide/flow-logs-cwl.html