[whatwg] Attribute value (double-quoted) state

[Mohammad Al Houssami (Alumni)]

Hello everyone,
Can someone explain wat is meant in the attribute value double quoted state in the tokenization specs : http://www.whatwg.org/specs/web-apps/current-work/multipage/tokenization.html#attribute-value-%28double-quoted%29-state
It says the below.
U+0026 AMPERSAND (&)
Switch to the character reference in attribute value state<http://www.whatwg.org/specs/web-apps/current-work/multipage/tokenization.html#character-reference-in-attribute-value-state>, with the additional allowed character<http://www.whatwg.org/specs/web-apps/current-work/multipage/tokenization.html#additional-allowed-character> being U+0022 QUOTATION MARK (").
What does the additional allowed character mean? It says the following:
The additional allowed character, if there is one
Not a character reference. No characters are consumed, and nothing is returned. (This is not an error, either.)

It didn't make any sense to me. Im still a beginner and know very few things about this and im trying to build a parser.
Any help or explanation would be very much appreciated.

Thank in advance
Mohammad