The message you see is a specified WIP limit. It’s an acronym, which stands for Work In Progress. By defining the WIP limit you set up the minimum and the maximum number of tasks that can be ‘In Progress’ column simultaneously. 

To set up the WIP limit, please, click on the three dots menu of a column and choose ‘Change WIP limit’.

Then you’ll see the pop up where you can set up the min and max number of tasks. Also, you can remove WIP limits on it.

Once the WIP limit is defined, you will be visually warned that you need to add more tasks to a column or if you reach the limit. 

Please note, if you have set the WIP limit of the composite column, tasks will be counted from the “In Progress” sub-column. 

You can read more about WIP limits and its usage in the Section “Agile 101”.