Word
Client id and client secret
A client id and client secret are the pair of values that identify an app you created: the id says which app it is, the secret proves you are the one running it.
Also called: client id, client secret, app credentials
When you make an app for your own store you get back two values instead of one password. The client id names the app; the client secret proves it is really you running it. Your app swaps the pair for a short-lived token by itself, about once a day, you never see or handle that token.
You meet them once, connecting your store: your agent saves both into the app without showing the secret, or you type them into its Setup page. Either way they stay on your machine, never in a chat.
The secret is the half that matters. If it ever gets out, replace the pair rather than tidying up the file it was in.