Baptism by Proxy and What does that Mean?

As I have been putting together Family Group Sheets for my Mother-in-Law, I noticed that some of her ancestors were baptized by Proxy between 2002 and 2010. I found this odd because my Mother-in-Law's family was LDS, came over with Brigham Young and even founded and settled Panguitch and Garfield, Utah. SO, why I ask you, would they be Baptized by Proxy if they lived and died a member of the faith? Definitely food for thought for the day.