Hello, and thanks for the question.
The call of the nightingale is used as symbolism for night and that of the lark for morning. For fear of his life, Romeo knows that he must leave the bedchamber of Juliet before morning and recognizes the call of he lark as signifying the break of day. Juliet, wishing that Romeo would linger a little longer insists, " It is not yet near day. It was the nightingale, and not the lark,..."
Hope this helps!