Learn about the Secrets Vault page:

Secrets let you add sensitive data (e.g. API keys) and use them in your workflows as personalization tokens.

Clicking "Add secret to vault" brings up a dialog box allowing the definition of a name/value pair.  

Entries can be removed by hovering over them and clicking the trashcan icon. The full value of a secret cannot be viewed in the UI once it is set.

For information on how to create a secret, see: Create a secret.