Jenkins uses this access key ID and secret access key to interface with Amazon EC2. If you have already signed up on EC2, you can obtain this from here. Otherwise, you need to sign up to Amazon Web Services to get one.
If no access key is set, implicit authentication mechanisms are used:
AWS_ACCESS_KEY_ID
and AWS_SECRET_ACCESS_KEY
(RECOMMENDED since they are recognized by all the AWS SDKs and CLI except for .NET),
or AWS_ACCESS_KEY
and AWS_SECRET_KEY
(only recognized by Java SDK)