What are hidden inputs?

[Lerc]

Hidden Inputs are inputs that do not need to be explicitly provided, They behave as inputs for the purposes of the execution function declared in the FUNCTION field of a node class.

The values for the hidden inputs are automatically filled out.

https://github.com/comfyanonymous/ComfyUI/blob/77c124c5a17534e347bdebbc1ace807d61416147/execution.py#L32

        h = valid_inputs["hidden"]
        for x in h:
            if h[x] == "PROMPT":
                input_data_all[x] = [prompt]
            if h[x] == "EXTRA_PNGINFO":
                if "extra_pnginfo" in extra_data:
                    input_data_all[x] = [extra_data['extra_pnginfo']]
            if h[x] == "UNIQUE_ID":
                input_data_all[x] = [unique_id]

[chrisgoringe]

Thanks - merged this into another answer!