In short, it means "If you will patiently listen to this story which we will soon act out for you here onstage, what I have not explained here, we will show you in our performance."
Let's break it down:
"The which" refers to the lines before. That is, the story of the love and death of Romeo and Juliet, and the feud between the Capulets and Montagues.
"If you with patient ears attend" means "If you will patiently listen"
"What here shall miss" means whatever has been missed, or not completely explained, by this prologue.
"Our toil" is the work of the actors in performing the play.
"Shall strive to mend" means that the performance will mend, or fix, any gaps in the story. The performance itself will explain any ideas that have been missed by the statements in the prologue.
Thus, the line means:
"If you will patiently listen to the upcoming performance, all the details that the prologue may have left out will be shown onstage by the actors in this play."