Flash Forward spins out of the event of the Heroes in Crisis story arc which spun out of the events of Flash War which kinda loosely spins out of the events of DC Universe Rebirth which ties back to Flashpoint only to the point that Wally West was lost (errr, editorially forgotten about) when DC's New 52 publishing initiative began in 2011. Why all history? So it seems... Wally West is finally getting a chance to be... himself (or some version of himself) again... after all these years.
If you didn't read any of those stories or don't really know Wally West terribly well, you may just think that this is a down-on-his-luck guy who feels responsible for the deaths of some of his friends and fellow heroes, and by the way, why the heck is he getting wrapped up into this journey to save the Earths of various Universes in the Multiverse? What makes him so special? Unfortunately, to read this story for its full effect, you might need some background knowledge.
A bit of background (that you can skip if you already know)... After Crisis on Infinite Earths of 1985-1986, Barry Allen "died" and Wally West took on the mantle of The Flash until Flash Rebirth in 2009-2010 when Barry returned. In those 25 or so years, many (and some of the greatest) stories were written about The Flash with Wally as the main character. And like so many DC characters, he had his family as central to his character and personality. He was married to Linda Park and had twin children, Jai and Iris. These 25 years (and his entire existence starting in 1959) were completely lost and unaccounted for after Flashpoint marked the beginning of the New 52 in 2011. It wasn't until DC Universe Rebirth in 2016 when Wally finally reappeared and has literally been trying to regain memories of himself. But in Flash War (2018), things took a turn when Wally went over the line at the newly realized loss of his twin kids. He was then sent to Sanctuary during Heroes in Crisis (2018-2019) only for things to go even worse for him as his power spikes and others are killed in the aftermath. And this is where Flash Forward begins.
A final thought before getting to the story... IMHO, DC has not really had a good plan on what to do with Wally since they brought him dramatically (and awesomely) back in 2016. That was an amazing moment where Barry Allen finally REMEMBERED Wally and Wally was pulled back out of the Speed Force and back into reality. But since then, Wally, IMHO, hasn't really had much to do. He's pretty much still depressed that Linda Park doesn't know him and he doesn't know where his kids are. So, for 4 years he's just been moping around? DC brought back Wally to mope around? I think this story finally gave Wally some direction that looked forward rather than always looking back.
As for this story itself, it does take a few issues to establish why the Tempus Fuginaut has chosen Wally for this mission and that he's the ONLY one who can accomplish this mission. And those issues read more like an abridged version of Multiversity (by Grant Morrison) or Multiplicity (Superman, by Peter J. Tomasi), but from a Flash perspective. It's kinda fun, but kinda, huh? Flash visits this Earth, does his thing. On to the next Earth... The story really only picks up in issue 4 when he meets an alt-version of Linda Park and we see Jai and Iris. Then the overarching plot finally comes into view and the drama of making a choice between (avoiding spoilers) something he wants and something everyone else needs comes front and center. The aftermath of this decision has an epilogue which is interesting as a review of the history of the DC Multiverse, but also as what it may have tried to be--as so many others have as well--that is, a launching point for the next publishing initiative for DC.
I think Scott Lobdell wrote this epilogue in the same vein as Geoff Johns wrote the end part of Doomsday Clock with this setup to bring in a new generation of heroes and stories that fully embraced the past (hence all the ink spilled on the history of DC storylines), but set to go forward with a new status quo for, in this case, Wally West, to carry on into this new era. Unfortunately, the 5G initiative died as the Pandemic hit and staff cuts hit DC publishing hard. Almost the most disappointing part was how it instead pivoted at the last moment to point readers to Dark Nights Death Metal to continue the story.
If you made it this far in reading this post, I would say that if you are not well-read up on Wally West or recent Flash storylines like Flash War or Heroes in Crisis, this book will likely not carry your interest enough to get you to the end in any coherent manner and final reveals will mean almost nothing without understanding the depth of storytelling that Wally West and his family was a part of that was simply "erased" from existence for almost 10 years. That said, if you are a fan of the Flash and want to keep up, it seems like this is a big change for Wally going forward and could even be an entry point into the Dark Nights Death Metal story arc.