The help explains it quite nicely
In this field, you can enter a date formula to indicate a safety lead time to be used as a buffer period in case of delays in the production or purchase replenishment lead time. When calculating order and order proposal lines, the program will add the safety lead time to the lead time. This is true for forward and backward scheduled orders. The specific date fields that are affected on the order (or order proposal) lines are as follows:
On the production order line, the Ending Date + Safety Lead Time + Inbound Warehouse Handling Time = Due Date.
On the purchase order line, the Planned Receipt Date + Safety Lead Time + Inbound Warehouse Handling Time = Expected Receipt Date.
Note that the safety lead time is ignored between items on a multi-level production order.