What is a walk-in?

My wife recently was asking me to tell her more about what a walk-in really is? I told her that around the people I was raised it meant that the soul residing in a body was through being on earth and that for some reason or other, God allowed another soul to take that souls place in the same body without the body dying. I told her how my mother felt that my grandmother, her mother and the one that mostly raised me while my mother was working a lot became a walk in shortly before I was born when she had a stroke. She then was in a coma for two months and when she woke up my mother said that her mother who was a very nervous thin person became someone different who was calm and who gained weight and was relatively happy to raise me for her when she worked. My mother said that both souls were very spiritual and religious and prayed a lot for everyone in our family including me. My grandmother was singing "Hark the herald angels sing" one Christmas in 1950 when I was two when Archangel Michael and his band of Archangels appeared in the room and healed me from whooping cough. So, if my mother was right since I didn't meet the first one, I was raised by a walk-in who was a very spiritual person and helped keep me alive spiritually and physically until I could take care of myself in all ways. I can't really say for sure because I wasn't there for the first one. But my mother says after the stroke her mother was a completely different person with a completely different demeanor and emotional disposition and then the stroke happened at age 60 and then she lived until she was 90 years old and passed away in Seattle in 1978 when I had just turned 30.