They talk about the meaning of the old prophecy on the library window. The old prophecy had only six lines:

When a golden girl can win Prayer from out the lips of sin, When the barren almond bears, And a little child gives away its tears, Then shall all the house be still And peace come to Canterville.

what does these lines means??

The ghost had requested Virginia to release him from the state that he was in. In order to do that she would have to weep for his sins because the ghost did not have any tears and pray for his soul because he had no faith. Then, if she had always been sweet, good and gentle, the angel of death would have mercy on him. Virginia would come across fearful shapes in the darkness and wicked voices would whisper in her ear but they wouldn't dare harm her because of her purity. The purity of a little child was far more powerful than that of Hell. The prophecy foretold that a golden girl who was pure of heart would be able to bring about salvation for the ghost and he could attain peace only then.