The maximum amount of Work-In-Process (WIP) that can be pulled in for processing / implementation within a workflow. This is a limit on the total amount of items that can be in process.
The purpose of having a WIP Limit is to increase the focus on the flow of value. Due to the limits on WIP, the remaining work items can be either Waiting to be Picked up or Completed (Done).
In a Scrum environment, to improve the flow of value delivery, it's recommended that the number of work items in progress may be lesser than the number of developers available to execute the work items.