Surely thine hour has come, thy great wind blows,
Far-off, most secret, and inviolate Rose.
(William Butler Yeats, "The Secret Rose")
In order to understand what I'm about to tell you, you need to know that the Witches' Goddess is known among witches, somewhat cryptically, as “the Rose.”
(If you can't see why that would be, then what kind of a witch are you?)
Hence the phrase sub rosa, “under the rose,” meaning confidentially.
When someone tells you something “under the rose,” it's not to be shared with outsiders. When told this, by listening, you thereby accede, as if you had sworn an oath.
The Craft is hedged about with roses. (I mean here, of course, the original rose, the rose of five petals.) The pentagram, of course, is known as the “witch's rose.”
Some things are of the Rose, not meant for others' ears. You may be told “This is under the Rose.” In season, a rose in bloom may be held up silently, and laid upon the table. A rose may be drawn with the finger in the air, or over the lips. All these forms are binding.
Under the Goddess, under the Rose.
Be told, my friend.
Be told.