When you publish comic books all set in a single shared universe, continuously, for sixty years, it is inevitable that continuity errors will arise.
DC Comics have made several major efforts to cut the Gordian Knot of continuity problems and ludicrously complex backstories which their characters suffer from. Hypertime is the third distinct attempt, following 1985's Crisis on Infinite Earths and 1994's Zero Hour.
Hypertime was introduced in The Kingdom, a comic book event from 1998. The basic premise of hypertime, as it was explained to the heroes of the DC Universe, is that time is not just a single straight timeline, or even a straight line which branches off into the occasional alternate future. Instead, it is a river, with "hypertimelines" continously diverging off from the main flow and merging into it again. The result, as DC Comics explained to its readers, is that everything is true, to a greater or lesser extent. For example, every past history of Superman, given in the Fleischer Superman cartoons, the feature films, the various eras of comics, the animated series, and so on, and so on, can be held to be accurate. Elements of Superman's origin which are constant across all portrayals - like the planet of his birth being named Krypton - might be places where these hypertimelines are all merged together. Meanwhile, the precise nature of his childhood in Smallville - did he adventure as Superboy? Did he know Lois Lane or Lex Luthor as a child? How much of his Kryptonian heritage was he aware of? - varies in the telling, and therefore takes the form of several equally valid hypertimelines running alongside each other, but all feeding into what Superman is today: the big guy in blue who defends Metropolis from supervillainy. In effect, to some extent there is a Superman archetype (river) consisting of certain inviolable facts, but the details change according to what flavour of story a storyteller is aiming for, and what medium he/she is intending to use.
In theory, this is brilliant: there is no longer any such thing as a continuity error. If Nightwing breaks his index finger in one story, but in one panel of that story it's his middle finger which is bandaged? Hypertime: one for each broken finger, merging back into the same story in the end because in the long run it's of no consequence. How come Batman is still in his thirties, having been crimefighting continuously for almost seventy years? Hypertime can surreptitiously modify contradictory dates, merging the stream in which Batman debuted in 1940 and the stream in which he debuted "fifteen years ago" or so. Trying to figure out Sandman and Hellblazer, which are nominally set in the DCU but only very loosely connected to it in practice? Job done.
But in practice, hypertime fell flat. For three reasons.
The Kingdom sucked. It was a lousy, boring story. It had bad art. It had no point. It was unmemorable - indeed, better forgotten. And above all it managed to severely tarnish the legacy of the critically acclaimed Kingdom Come, to which it purported to be a sequel. Right now, few people remember The Kingdom, for good reason, and so not many people even know hypertime exists as a concept.
It didn't make for good stories. Or, indeed, any stories. There was one Flash arc which featured a slightly modified version of Wally West from an alternate hypertimeline falling for someone in the mainstream DC Universe - but never being able to be with her, because crossing hypertimelines was abominably difficult and threatened to do something horrible to the fabric of spacetime and what have you. Apart from that... well, what ideas do you have? Hypertime was not a source of inspiration and few writers picked it up. So, once again, it came to be forgotten.
And lastly, it was worthless from a practical standpoint. If everything is true, then, to look at it another way, nothing is. If you can pick and choose which old Batman stories Batman is going to be referencing on his current caper, and which ones are going to be ignored, then none of them have validity, and nor is there a definitive history of Batman. Fully embracing hypertime would mean simultaneously abandoning the maintenance of a rich and internally consistent DC Universe.
But... that's why I like it. Since internal consistency is infinitely preferable to waving a magic wand and saying "ehh, it's all true", the invocation of hypertime to explain continuity errors is a weapon of last resort for readers and simply no longer on the radar for writers. That's how it should be. Hypertime was established to be "the biggest secret in the universe" when it was introduced, so for it to appear once and never again - indeed, for everybody involved to forget all about it - is perfectly consistent with this. Hypertime is not a source of inspiration for new stories and it's certainly not a story in itself and it may never appear again. But it is still canon, and its explanatory power, as demonstrated in the examples above, is necessary and valuable to those of us who care. While DC Comics attempts to build a totally rock-solid continuity, hypertime is there quietly filling the smaller gaps.