"Mammy" in Stowe's work is actually Aunt Chloe, who is Tom's wife.  She is his wife as well as the mother of his children.  Like Tom, she is a slave, a part of a culture that renders her without power.  She and Tom work on the Shelby plantation.  For Stowe, her use of Mammy is significant.  It helps to show how slaves were a part of families and lives in which love and the presence of care was intrinsic to their lives.  They were not, as many Southerners saw them, property or objects.  The love shared between Tom and Mammy is real.  When Tom is sold off of the Shelby plantation, Mammy is given the opportunity to work as a baker.  The hope here is that she would be able to make enough money to buy Tom's freedom in order to reunite their family and enable husband and wife to live together.  Again, this is deliberate on the part of Stowe in order to show how a wife must sacrifice in order to save her husband from the cruelty of the slavery institution.  Mammy's role is significant because she has to do what no wife should ever have to do.  The fact that she is unable to save her husband becomes testament to the ultimate cruelty and horror of slavery.