A Floating Deadline is a kind of deadline where, because the mod is absent or otherwise away from the computer, the players can continue posting and voting past the normally scheduled deadline. The functional deadline, then, is whenever the mod is able to return to check the thread.
For instance, if a mod sets a floating deadline for 11 PM EST, then barring a hammer, the day will end no sooner than 11PM EST. But if the mod is AFK, then every minute past 11 is a free minute to vote for an execution candidate. However, every minute past 11 that the town has not successfully executed somebody, they take the risk of not executing anyone, as the mod could check the thread at any moment without warning. Maybe he or she will show up at 11:05, or maybe 12:15 AM, or maybe even 3:30 PM the next day; it is a gamble.
This kind of deadline is generally the exception, and a deadline should always be presumed hard unless specifically stated otherwise by the mod.
They are also called Schroedinger's Deadline.