Meaning
A rainbow baby symbolizes hope, as well as healing for parents who have been blessed by a healthy baby following a miscarriage or infant loss. It brings great joy after a very difficult time and these babies are treasured because they’re like a reminder that after the storm and darkness, something beautiful like a rainbow can appear.
Source
https://themindsjournal.com/what-is-a-rainbow-baby/
Wikipedia
Further Reading:
https://www.webmd.com/baby/what-is-a-rainbow-babyistics.html