lubobill1990 Cookie is required for user to login to flarum. Do don't need to care this grade C.
That's correct, but not in this sense. What this refers to is the delivery of assets from a domain that does not use cookies at all - it has no bearing on the flarum session cookie. In this case, the page speed insights are suggesting that you deliver the assets needed to support flarum (in terms of JS, CSS etc) via a domain that does not store cookies.
A CDN is a typical example of this. Again, it's a recommendation, and certainly not mandatory.
