WIP (Work In Progress) indicates that a mod is not completely finished yet. It is usually indicative of an alpha state and warns that not all features have been finished yet. WIPs also tend to have more bugs than finished versions of the mod. However, just because something is not finished does not mean it is not worth trying. Trying a mod out before it's fully released is a good way to see if it's something you might be interested in, in the future.