What is Goods Receiving Tolerance?
Defining a tolerance percent gives you the flexibility to receive the goods more than the qty ordered. Sometimes, the supplier sends out more units than ordered. Going by the rules, the GRN won't be allowed to do for qty more than ordered qty hence, tolerance %age is defined at PO level which gives you the liberty to accept/receive extra goods.
With the above setting, if ordered qty of an item is 10, you can receive up to 15 units.