If you use an API key to access a secure service, or need to use a password to access a protected database, you'll need to provide these “secrets” in your R code somewhere. That's easy to do if you just include those keys as strings in your code — but it's not very secure. This means your private keys and passwords are stored in plain-text on your hard drive, and if you email your script they're available to anyone who can intercept that email. It's also really easy to inadvertently include those keys in a public repo if you use Github or similar code-sharing services.
To address this problem, Gábor Csárdi and Andrie de Vries created the secret package for R. The secret package integrates with OpenSSH, providing R functions that allow you to create a vault to keys on your local machine, define trusted users who can access those keys, and then include encrypted keys in R scripts or packages that can only be decrypted by you or by people you trust. You can see how it works in the vignette secret: Share Sensitive Information in R Packages, and in this presentation by Andrie de Vries at useR!2017:
To use the secret package, you'll need access to your private key, which you'll also need to store securely. For that, you might also want to take a look at the in-progress keyring package, which allows you to access secrets stored in Keychain on macOS, Credential Store on Windows, and the Secret Service API on Linux.