Work in Progress - aka WIP - is the work, that has been started, but has not finished, yet. The term originates in Supply Chain Management and is an important concept in Lean Production.
Within software development, WIP is not obvious, unless extra effort is undertaken and tooling is in place to visualize it. A Kanban Board is one of such tools.
Minimizing WIP is extremely important for efficiency, as ongoing work affects other ongoing work through Context Switches