The title is just Mary's name: Mary Poppins. It's not too hard to figure that one out. The story is focused on solving the Banks Family's dilemma, and Mary is the person to make this happen. She's the puppet-master, the magician pulling all the strings, leading the kids on adventures and guiding Mr. Banks back to happiness and kindly fatherhood.
Without Mary, there's no movie. It's as simple as that.