It's the hand-sign that, in the language of the Mysteries, means: Mystery.
The ritual has just reached its high point. The Mystery has been revealed.
How do you communicate: What you have just seen is a mystery, not for revelation elsewhere?
You can't simply say, Don't talk about this.
Then you'd be talking about what shouldn't be talked about.
No. Instead, you give the gesture. You say it with signs.
The finger raised to the lips. Silence. Not to be spoken of.
We all know the gesture. We all understand what it means.
Behold: the Mystery.
Shhh.