It’s a log of all traffic in your VPC. Flow logs are not turned on by default. You have to create one explicitly by selecting the “Flow Log” tab when viewing a particular VPC – and going thru the “Create Flow Log” process.
There are lots of limitations on flow logs and they can cost money. See the following references.
References:
https://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/flow-logs.html
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