In many ways of working, setting a limit on concurrent work is desirable to make sure that a process runs smoothly. In the Kanban methodology, for example, this is done by limiting the number of cards allowed to exist in a specific column -- the work-in-progress limit or WIP Limit. This is what the WIP Limit app allows you to do.
The WIP limit app is enabled on individual boards through the App Store in the hamburger menu. Just click enable and it will enabled for that specific board.
After enabling the app you will have the choice to set an Enforced WIP Limit. If this is set users will not be allowed to move cards to columns exceeding the WIP Limit set on that column.
To set the WIP limit of a board column, click or tap a board column header to bring up the column menu, check the WIP limit checkbox, and enter the desired number of cards allowed in that column. Setting a WIP Limit of zero is equal to disabling WIP Limits for that column.
After a WIP limit has been set on a column, a counter will appear in the column header. It shows the number of cards currently in the column and the maximum number of cards allowed. The box will be colored green unless the limit has been reached, in which case it will be red (unless you've enforced the WIP Limit, in which case you will not be able to exceed the set limit).