Lady Capulet has been looking for Juliet and wants to share important information with her. When she finally finds her, Juliet shows her mother the required respect and courtesy that was tradition at the time. By formally addressing her mother as 'madam', Juliet is indicating her respect for her mother.
The term also suggests obedience, for a madam takes charge of her servants and instructs them what to do. In this regard, therefore, Juliet respects her mother's authority and is ready to follow her instructions. 'What is your will?' emphasizes this, since Juliet wishes her mother to instruct her and she will do what she requests - it is the same address a servant would use when speaking to a master or madam.
It is therefore clear that Juliet knows her place and is obedient to her mother. She does not question why her mother is looking for her, but shows a willingness to carry out whatever instruction her mother issues. Juliet's courtesy clearly indicates that she unquestionably recognizes her mother's authority and that she would obey her.
This obedience is further emphasized later when Juliet is instructed by her mother to consider getting married and become acquianted with Paris since he has expressed an interest in marrying her. Juliet does not challenge her mother's request and says:
But no more deep will I endart mine eye
Than your consent gives strength to make it fly.
This suggests that Juliet will only allow herself to fall in love with or become endeared to Paris for so much as her mother would permit.