basically we all agree the baby represents voldemort - in some way.

psyduck explained it "It sounds like a replica of the thing in GoF, before Peter dropped it in the cauldron."
which can go on to mean that it is a weaker version of voldemort.

victoire said "I think the baby was like Voldemort's soul...undeveloped and tortured. I mean, you can't expect his soul to look great after being split up seven times."
which also explains everyones justification as the fraction of his soul remaining. because, our natural response to reading, is visualising the words.
so we picture a soul (which in the first place is intangible) but as a whole, and then we know he gradually tore it into 8 different parts, basically. each residing in different forms (horcruxes). seeing as harry and the others had destroyed all parts but 2 (voldemort himself and nagini) by the time kings cross came around, voldemort's life was deteriorating rapidly.

the story went from voldemort trying to kill harry, and then the next thing we know the story takes him to kings cross.
waking in an empty room only containing harry. and the gross baby.

the way i see it, that baby had been the part of voldemort resisiding in harry (for visualisation sake - lets say it was the scar)
now in this limbo stage;
Elisabeth: In the chapter of kings cross, are they behind the veil or in some world between the real world and the veil?
J.K. Rowling: You can make up your own mind on this, but I think that Harry entered a kind of limbo between life and death.

harry had finally rid himself of voldemorts soul which had been in him since age 1.
dumbeldore was saying , don't worry, it's beyond help etc etc.

other things regarding this;
the absence of his scar, glasses
prescence of dumbeldore
being in kings cross

as discussed earlier, people have said they think it was because harry wanted it to be so.
and JK confirmed this to be true. in an 'ideal' world he woulndt have the scar, not need the glasses, and in limbo between life and death, requires dumbeldore and his knowledge in order to understand the situation in its entirety, and to put at rest his concerns from the past (especially regarding dumbeldores life etc)

when harry returned to consciousness, he had at that time discovered that there were 2 and only 2 parts (of a whole) left of voldemorts soul - little did voldemort know!

harry realised voldemort had also been unconscious, however after reading over everyones thought i think maybe that could just be the avada kevadra curse backfiring - and not killing him because he had in fact made it less powerful (the magic may have been consumed by the horcrux in harry)
anyway voldemort created the horcruxes to ensure he could basically live forever. so an avada kedavra curse wouldnt have killed him, it would have just maybe turned him into a weaker, grosser version of himself!


hahah right i apologise that that was so large, but i think it all had to be summarised.

i was just trying to show that everyone who posted responses were basically right!