When you interact with AWS, you use AWS security credentials to verify who you are and whether you have permission to access the resources that you are requesting. In other words, security credentials are used to authenticate and authorize calls that you make to AWS.
For example, if you want to download a specific file from an Amazon Simple Storage Service (Amazon S3) bucket, the credentials that you use must allow that access. If your credentials aren't authorized to download the file, your request is denied. In some cases, you can make calls without security credentials to AWS, like when you download a file that is publicly shared in an Amazon S3 bucket.