Tolerance days are simply a buffer used on days for products to be received, this affects the actual due dates given to the Order Recommendation after running the MRP. This is a Optional setting and does not need to be used.
Tolerance days can be defined in the Planning Data tab of the Item Master Data screen.
1) From the Modules tab, navigate to: Inventory > Item Master Data
2) Click on the Planning Data tab and locate the Tolerance Days field: