What does it mean to send a short message to two swallows in the sleeve?

There is a short book in the sleeve, which I want to send to Shuang Yan.

In the Southern Dynasties, Jiang Yan wrote a poem "Miscellaneous Poems", which said: "There is a short book in the sleeve, and I would like to send two swallows." Legend has it that geese can pass books, so can swallows. But when the swallow came back, he forgot to bring the letter written by the wandering lover to the boudoir woman. The woman looked forward to it in the boudoir, expecting the swallow to bring her fragrant letters, but she was disappointed in the end.

or

I can't go with you, so please pass this book to the swallow. The flying of geese is a parting gesture, and Shuang Yan is always looking forward to bringing news of reunion.