This means that LJ will be non-existent. There will be no posting, no Friends page, no LJ mail, nothing. They plan to bring the site back up slowly, they are going to ease into it rather than open up a floodgate of traffic. Therefore posting might not be immediately available or the site could be slow to load for a while.
This will begin at 8:00 A.M. PST which for Brits is 4:00 P.M TOMORROW 17th November, 2008. And they estimate that it will take FOUR hours to move - but warn that it may take longer, best laid plans and all that.
If you're not sure what time this is in your part of the world The World Clock is a pretty good website.
For full details, including the new IP addresses (plus a note to say that both the Permanent account and A La Carte sales are to be delayed) go here.