WIP is a shorthand way of saying Work in Progress. A fanfic is a WIP if it has not yet been finished. A rough draft is a WIP if it has not yet received a final edit or beta reading. An artwork or website can be a WIP if it, too, is unfinished.
The issue of WIPs is important to Daria fandom as so many fanfics are left unfinished every year, the first few chapters or segments posted to a message board with no conclusion or resolution. Some unfinished fanfics have considerable influence over the writings of others, pointing the way to new shipper pairings or plot ideas. A written WIP might be forever left unfinished, but it can be cannibalized by the author to develop new stories. TAG's "Bipolar II" is an example of a WIP that was used to created several other stories, the original plot having been abandoned.
As a side note, it is not correct to speak of an "unfinished WIP," as that is redundant. There are no finished WIPs, either, only finished stories.