The first time I had ever seen or read the term was in one of *shudders* Doreen Virtue's *shudders* books called Earth Angels: A Pocket Guide for Incarnated Angels, Elementals, Starpeople, Walk-Ins, and Wizards (Wise-ones). By her definition 'Walk-Ins' are those who trade soul places. If that still isn't making sense think of it this way...a soul swap. When someone is horribly unsatisfied with their life and maybe plans on ending it or some other circumstance arises, a Walk-In soul will negotiate with the potential Walk-Out until the Walk-Out decides the terms are fair (this is all done on a deep subconcious level mind you) and then the swap is made. Supposedly by her viewpoint there are many souls awaiting on the other side that have jobs or duties to fulfill in other worlds. Instead of wasting a full grown mature human body (or whatever type of body it is that is at a mature age) souls often swap in this manner, so that another soul who has a purpose to fulfill can get to it immediately instead of waiting through infancy, childhood, tween, teenager, and then adult.
However I've also been informed by my beloved that he knew someone that this happened often to, where a soul would simply Walk-In, take over for a while, then give it back to the original soul owner of the body (considering that made any sense).
Overall this book that I've read to some degree I wouldn't recommend any of her books, especially this one. As a Ph. D. with a degree in Metaphysical psychology she should know better that fluffy, happy, warm and fuzzy just sounds very fake (least to me it does). Reading just the intro alone to this book makes me feel my teeth are rotting at an incredible rate with how much sugar she pours on. Also her whole concept of reincarnation seems a bit skewed. From this book she almost makes it sound like you cannot live/be reincarnated into every single one of these beings she lists (Walk-Ins being excluded considering I don't believe that to be a true rebirth, but we can argue that point if you'd like anyway). She almost gives me the impression that her mental view of the "The World of Reincarnation" is a very small one.
Okay enough ranting on her book. I'll let you all be the judge of her work.
"For many are called, but few are chosen" - Matthew 22:14 ver. New American Standard.
"Though my soul is born from darkness, it shall rise in perfect light, I have loved the stars too fondly, to be fearful of the night". Unknown
"On those nights that I gazed upon the full moon in longing it was always you that my heart was calling." - me