Access Control Lists (ACLs) help you manage access to your buckets and the objects within them. They are considered the legacy way of administrating permissions to S3. Why should you know about them? If you have to manage access to individual objects, then you would use an object ACL.
By default, when you upload an object to S3, that object is private. If you want to make this object available to someone else, you can set the object’s ACL to be public at creation time.