Access Keys are key-value pairs that are configured on the instance (very similar as you save yours on your computer ) and they are associated with an IAM account which has a policy that defines the permissions o

Access Keys will pick up those permissions and EC2 will use them.

Since they are stored on the instance it is not a very secure way to grant your instance access to other services (in case the instance is compromised, anyone can then use those access keys (it is as simple as typing cat ~/.aws/credentials)

Do not store Access Keys on your instances!