Apparently, Senator Feinstein sent an anonymous letter to the FBI, alleging something bad happened that dates back to when Kavanaugh was in high school. This is, basically, a delaying action and a threat that the FBI will end up investigating the nominee after he is on the bench. That would be embarrassing for everyone who voted for him, especially if whatever it is has significant evidence and is serious. And of course it's a mystery allegation, so there's no evidence to check one way or another. Congressmen are basically being asked to vote on an accusation alone, without evidence or details.
Senators should vote to confirm anyway. If the accuser is not willing to have his or her claims tested and publicly stand behind them, they should not be made at all. Certainly they should not be dispositive of a nomination decades later.