Word
Secret
A secret is any value that proves you are you, such as a key, a token or a password, and anyone holding it can use it as you.
Also called: credential
An API key, a database password, an access token: anyone holding one can act as you and spend your money, which is why it's handled differently from every other value in your project.
You'll meet secrets the first time the app needs to reach anything beyond your own machine. They live in an environment variable, read at startup, never written into a file you save into the project, never pasted into a chat.
If one does leak, deleting the message isn't the fix. Rotating is: replace the key so the leaked one stops working.