Snyder has said that everything will be explained. It may take a while to get to everything, though. Snyder is a slow burn kinda guy.
But basically, Alec Holland was a scientist who died in a swamp. Initially, everyone thought he became Swamp Thing, but in Alan Moore's run it was revealed that the 'Green' grafted his memories onto a creature made entirely of swamp vegetation. Alec was truly dead, Swamp thing just had his memories. In Blackest Night, Alec Holland was revived, I never read that, but I take it that all you need to know is that Alec is back, and has the memories of Swamp Thing. That's really all you need to know.
All I had ever read was Moore's Swamp Thing (lots came before and after his run) and that was years ago. I followed the story just fine.
btw in solicits for issue 4, it's revealed that this and Animal Man are both dealing with "The Other", and it's looking like the books are fairly closely linked. "The Other" looks to be a new counterpart to the Green (plant life) and the Red (animal life). Based on the mammoth creature and the undead critters in Animal Man seems to be related to dead things and decay. Normally, I'd be a bit irked about this. but Jeff Lemire and Scott Snyder are best buddies and are no doubt working very closely with eachother, and both books are excellent, so I'm looking forward to it.
http://ifanboy.com/articles/first-look- ... -december/
[Edited on 19/9/11 by Bamfette]