Do you really want to know this in advance of playing the game? I will admit that it is one of the more intriguing aspects of the game but it gets explained fairly well in the game. After an hour or so you will know everything you need to know about the baby.
Spoiler Time* – So if you really can’t wait, and want to know what this baby in a pickle jar is all about, I will explain enough so that it makes sense. If you have played the game and are confused as fuck about it, then this might clear things up. There will be no story spoilers here but I will explain everything about the baby and what the point of it is.
The baby is known as a Bridge Baby or a BB for short. They are attached to the world of the dead and they interface with you via the pod they are living in. There is definitely a moral issue around it all and Sam does seem to feel a bit uneasy about them being babies but they assure you that they are not actual children and would die outside of the pod.
The BB helps you spot the ghost like things you see in the trailer. That little clappy scanner thing around your shoulder is how you locate the ghosts known as BTs. These will kick your ass, so knowing where they are is important. The BB is how you know where they are.
Why didn’t they just make some sort of gadget to detect them instead of a baby? The reason for this is justified in the story, but ultimately it seems to just be a cool decision from a development perspective. I’m sure they could have come up with something else but a baby is very impactful and it has worked. Everyone has been asking me for weeks “what’s the deal with the baby”. At least now I can finally talk about it.