The safety stock is the stock that Optiply thinks it needs to deal with unexpected events.The safety stock is the stock you hold extra on top of what you expect to sell. You need this buffer to be able to react to unexpected events in the future. You can think of a peak in your sales or a new delivery that is delivered too late. This buffer depends on the category of the product, the variability in the sales history and the supplier's security of supply.
For example, a product in the A category from a reliable supplier that is sold 5 times a day will have a low safety stock. A product that is highly variable and has an unreliable supplier will have a higher safety stock.