A wip is basically an update of something you're making. In the community, they are most commonly art projects or replays in progress. Posted to get opinions.
WIP - Is an unfinished version of image, textures and etc..
Why showing an unfinished art you ask?
Because the other people can tell you what to add to your art
and if they see any flaws, they'll tell you.
I'd like to add some more information:
WIP stands for Work In Progress.
This can be anything, even non toribas-related stuff.
A player can post a WiP [let's say a head texture] in the request section [As an answer to a request] and tell the owner of the thread it's a WiP.
This means the artist will most likely post a more updated version of his head texture later on.