What is a/what do you mean by “rainbow baby”?

Don’t hate, just explain. It’s a genuine question and I know I could google it but I’ve been scrolling, reading and voting for over an hour and I don’t think I’ll be exiting the app anytime soon lmaoo

POST EDIT/UPDATE:

So apparently I’ve been misunderstood and I’m gonna make this crystal clear for some people: my “lmaoo” was not aimed towards what I’m asking about, it was aimed towards the fact that I was addicted to reading all these posts for over an hour (no joke, my boyfriend was starting to feel left out lol) and so I knew it’d be impossible for me to exit the app and google the question. Plus, it’s not like google always gives straightforward answers to begin with.

As for sounding “insensitive”- I asked a freakin question. And now that I know what it means, I’ll know to be nicer talking about it in the future. But how would I know that there’s a bittersweet/painful feeling/memory/moment behind this word without knowing what it means at all? Just saying.