Every deal in the Sales Pipeline has the estimated closure date set. As well as, every deal has the limited time of how long it can stay in the particular stage. When the deal is late to be moved to another stage or late to be closed (won or lost) the system starts notifying you about this.

If your closing deadline passes, you will see a red yellow/red bar underneath specific deal in the pipeline. The longer it is overdue, the longer and redder the bar will be.