It creates an ominous mood that hints at the conflicts of the play.
It recaps past events that happened in the play or offstage.
In this prologue, it begins by explaining that Romeo's feelings for Rosaline have gone and they are replaced by a new love. His new love returns his affections. However, she is his enemy. The prologue then talks about how Juliet is in love with her enemy. This is a recap of past events as well as hinting at the conflicts that come later in the play. The prologue reminds the reader that these two are not supposed to be in love and it will be a problem for them.