Some plugins require authentication information such as a password or API key to be able to work, e.g. to login to an FTP server or to update a DNS record. These secrets are historically saved in encrypted form in the .renewal.json files in the configuration folder.
There are also some global secrets, like the proxy server password and the smpt server password, that are stored in settings.json.
Version 2.1.17 introduced the secret manager to make it easier to re-use and manage
secrets for renewals. Also it makes it possible to protect those aformentioned global
secrets. The secret manager can be accessed from the main menu by going to
More options... > Manage secrets. There you will be presented with a list of currently
known secrets (if any) to update/delete them, and an option to add a new one. Each secret
has a unique URI like vault://json/mysecret which you can use in various places like
configuration files, command line arguments or script installation parameters.
Currently there is only a single backend for the secret manager, which is a .json file
in the configuration folder. The location of that file may be modified through
settings.json, for example if you want to share it between different
ACME endpoints. In the future the idea is to support more backends like Azure KeyVault.