Theologically it is supposed to mean that Jesus took on the ancient Judaic role of scapegoat and sacrificed himself to himself to appease his own outrageous levels of bloodlust so completely there would never need to be another blood sacrifice to atone for sins which means every human being born since the crucifiction and resurrection no longer have to atone for "original sin" (unless they're Catholics) and we are all "saved" because god committed suicide (in a roundabout way) and brought himself back to life. Also relates to the story of Abraham being ordered to sacrifice his son Isaac and then being told not to at the last moment (this event meant that god was no longer accepting blood sacrifices which had previously been the only way to appease him).
Essentially, it's a rather bizarre and frankly horrific story which makes no sense whatsoever outside of the context of people for whom the concept of animal and human sacrifice to appease their god(s) is so ingrained that the idea of it ever ending and no longer being required by their particular deity is so noteworthy it is enough to start a new major world religion.
(apologies if i got some of the details wrong (not 100% sure on the original sin bit to be honest as it's been a while since i spent much time on theological matters) but am sure our more knowledgable folk will correct me on such things and the gist of what i say is correct)
PS: How this all affects people born prior to this (non) event is somewhat vague in similar ways to how people who were never in a situation where they could possible know about Christianity and so could never be baptised or confess or convert - or even be charitably burnt at the stake to save their heathen souls - is also somewhat vague. Let's be honest here, almost all of this kinda stuff is vague because it is ancient mythology of peoples who had no concept of there being a world or people outside of their incredibly small and limited perspective and locale. Does kinda make you wonder how this stuff ever got to be a major world religion... then you remember all those conquests and crusades and horrendously brutal "conversions" or entire populations - basically anybody who expressed the slightest doubt was killed (usually in especially horrendous ways) so their descendants never made it this far. It's no wonder so many religious folk have a problem with Natural Selection - they far prefer the deeply unnatural selection process of religious eugenics.
![:\](https://bluelight.org/xf/BL_Images/Orig_Emoji/wrygrin.gif "wry grin :\")
if only it had been written this way in the bible, perhaps we would have been spared the last 2000 years of superstitious bullshit.