It's really not.
One of Xehanort's incarnations went back in time so he could collect all the various vessels he'd established throughout the series' history and assemble them in one place.
That's it.
One of the "rules" for going back in time is kind of weird (you have to lose your body to do it), but when KH2 defines the difference between Heartless and Nobodies it explains that Heartless don't have bodies. Given that, it makes sense that the antagonist of KH1 was the one to go back in time, since he's the incarnation of Xehanort that's a Heartless and therefore lost his body.
I don't think there's anything else you need to be able to take away to "get" the time travel.
I've read this post like three times and still don't understand it.
"KH is extremely confusing and that's why it has a bad story" is just short-hand for how much people dislike the series' story but put in a form of criticism that seems more objective. There are many more issues with the story than the confusion.
-Sora is an extremely unlikable character in KH2.
-KH thinks it's very deep but has nothing whatsoever to say. KH has the same major flaw of Star Wars in never defining what "Light" and "Darkness" actually are, but KH basically is navel gazing most of the time. There is no insight or even coherence in the series' rambling about Light and Darkness and the series seems to have no idea what it would even define as Light or Darkness or what that would mean but it keeps talking about those things incessantly.
-The Disney story sequences are robbed of joy in KH2 and are just horrible because Sora is often just along for the ride with his story and the Disney characters story having nothing to do with each other. Square puts together a very low production value (with poor dialog and really bad directing) version of a Disney movie and then shoves together a story about Sora chasing the Heartless. Sora leaves most of the Disney worlds having gained no knowledge or insight about life and is the same going out as he is coming in. Meanwhile, Sora has no impact on the Disney story, making the plot boring because we know what's going to happen and making you feel like the random hanger on in the Disney story that you are.
-There are way way way way too many cutscenes.
-All the clones do legitimately get confusing to keep track of at times (I was baffled when Replica/Data Riku showed up, wondering why Riku got young again in the trailer) but they're also just lame and boring. A villain that is just Sora or Xehanort but a clone is incredibly uninteresting at this point.
-The forward momentum in the plot often doesn't occur within Disney stories, making plot progression feel extremely slow and making the Disney worlds feel like filler when they should be most of the point.
I only take issue with this small part of your second point, SW does explain the difference between the "Light Side" and the "Dark Side," to an extent. The Light Side is essentially about the the natural order of things, it's fueled by traits and emotions such as selflessness, serenity, and innocent curiosity. As Yoda states, you know the Light Side when you are "calm, at peace...a Jedi uses The Force for knowledge and defense, ever to attack." On the other hand, the Dark Side is about selfishness and all its accompanying traits and emotions such as anger, aggression, possessiveness, quests for immortality, etc.
The real problem lies in the whole, "there must be balance" which the TLJ sort of muddled further. We're told there should be balance between the two forces, but we're never told a straight up answer as to what does balance mean? Does that mean the Dark Side needs to exist? What exactly is the Dark Side then, is it a natural occurrence or a corruption of The Force caused by man's selfish use of The Force? Before it appeared the latter was the case, but TLJ makes it seem as though the Dark Side is a natural element found in nature, particularly violence and death. And thus, it must exist in order for balance to be achieved.
Like, why are baby Porgs being killed by naturally rising tides part of the Dark Side? I don't know. So, essentially we know what the two Sides entail, what fuels them, we just don't know what inherently causes them to exist in the first place.
