AWS Key Management Service (KMS) is a basic AWS service that can create and manage cryptographic keys. KMS provides a single control point from where you can manage your keys. 

AWS Secrets Manager is a managed service that lets you quickly rotate, manage, and retrieve any credentials, API keys, or secrets.

Secrets Manager will encrypt and store your secret, transparently decrypt them, and return them to you in plaintext when you request them. One of the main benefits of Secrets Manager is that you no longer need to hard-code your credentials in code.