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.
WIP can be obvious - like a pile of parts before a machine in a factory - or invisible - like in software development. It is always extremely beneficial to make WIP visual, for example through a Kanban Board, so that it can be understood what needs to be done.
Minimizing WIP is extremely important for efficiency, as ongoing work affects other ongoing work through Context Switches. Also the more greater the pile of accumulated WIP, the less clear becomes the actual Throughput and thereby the effective Lead Time (“we are working all the time, but never get anything finished”).