Answer and Explanation:
The double meaning of "to fall prostrate here" is related to what Juliet is doing now and what she will do later that night. Juliet, who is in love with Romeo, is being forced by her father to marry someone else. She is instructed by Friar Lawrence to pretend regretting her behavior, to pretend that she agrees to such marriage, and to fall prostrate. She does fall prostrate - she falls to her knees while begging her father for forgiveness. Let's keep in mind that "prostrate" means lying stretched out on the ground with one's face downward.
However, Juliet will also fall prostrate later, when she drinks the potion Friar Lawrence has given her. The potion will slow her heartbeat down, making her look like she is dead. There is a certain irony to this, as if Juliet is teasing her father, foreshadowing what will take place soon but without any chance of him grasping that meaning.
