When your bot receives a message, Telegram sends an update object to your bot. The update contains information about the chat, the user, and of course the message itself. There are numerous other updates, too: https://core.telegram.org/bots/api#update

A function of this type is passed as the second parameter to all middleware. Invoke it to call the downstream middleware and pass on the control flow.

API call transformers are functions that can access and modify the method and payload of an API call on the fly. This can be useful if you want to implement rate limiting or other things against the Telegram Bot API.