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.
Juliet is formally asking her mother what she wants her to do. This demonstrates obedience because she seems ready to do what her mother asks.
In Act 1, Juliet is very obedient. She is mostly respectful to her mother, and considerate to her father. When she asks her mother what she wants, she does it in a formal and polite fashion.
When Lady Capulet asks Juliet how she feels about marriage, her response is reserved and polite.
It is an honour that I dream not of. (Act 1, Scene 3)
Juliet does not want to marry. She thinks she is too young. But she goes along with the discussion because she wants to be polite and because in her culture parents decide for their children. Juliet has no say.
Juliet’s obedience is not a bad thing on its face, but it has tragic consequences. If her parents had not tried to enforce their will on her, she and Romeo might have still been alive.
In Act One, Scene Three of Romeo and Juliet, Lady Capulet wishes to speak to her daughter and orders the Nurse to call her forward. Although Juliet is a little slow to arrive, resulting in the Nurse having to call for her several times ("What, lamb! what, ladybird! / God forbid! Where's this girl? What, Juliet!"), Juliet snaps to attention once she learns that it is her mother who is waiting for her.
Juliet then respectfully asks Lady Capulet, "Madam, I am here. / What is your will?" In this way, Juliet is presenting herself as eager to listen to her mother and prepared to obey her wishes. Note that Juliet does not refer to her mother as "mother" or reference her in any other sort of personal or endearing way; rather she uses a term that might normally be reserved for servants to use for their masters. This is Juliet's way of acknowledging her mother's superiority in the household and her own status as a daughter who must obediently defer to the desires of her parents. Knowing that this is what is expected of her, Juliet is prepared to "serve" her mother and to acquiesce to her needs. As we soon find out, Juliet is even willing to consider marrying a complete stranger if this is what her mother commands.
Finally, it's worth noting that this sort of strangely distant mother-daughter relationship is in stark contrast to Juliet's relationship with the far more emotional, overly-concerned Nurse, who seems to act as a confidant and maternal figure for the young girl. Although Juliet may not be able to outwardly disobey her real mother, she certainly exercises some teenage rebellion by involving her Nurse in her romantic escapades as the play progresses. Thus, when Juliet finally does try to evade her parents' wish for her to marry Paris after her secret wedding to Romeo, she is greeted with shock and threats of being disowned.