In the movie she seems to sit on a cloud, exhibits magical powers, and is sent to help a family come back together. She seems to be operating on a mission of sorts as she helps a lot of people. Bert in the movie is actually multiple characters in the stories which Mary Poppins has been associated with. She also doesn't appear to age, as the grownup Bert has known her presumably since childhood (and they are roughly the same age).
I always assumed she was some heavenly being (such as an angel).
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