Stock rotation is the practice, used in hospitality and retail, especially in food stores such as restaurants and supermarkets, of moving products with an earlier sell-by date to the front of a shelf (or in the cooler if the item is on repack so they get worked out before the new product), so they get picked up and sold first, and of moving products with a later sell-by date to the back.
Most, if not all, packaged perishable food products, will have either a sell by date on them or a display until date; in practice, these are exactly the same thing. After this date, it is either illegal for the store to sell them (this is the case in the United Kingdom) or the quality will have deteriorated to the point at which nobody will buy them. In either case, they cannot be sold.
If a product is still on shelves after its sell by date, it will have to be thrown away (recorded as wastage), which is both costly and wasteful to the store (suppliers must be paid even if stock is not sold). Therefore, it is imperative that sell by dates are strictly adhered to, and that products which will perish earlier be sold as quickly as possible.
Shoppers, on the most part, will simply walk up to a shelf and take the frontmost box of the product they are looking for; this is especially true if they are in a hurry. They will generally also, unless they are specifically looking for a product that will last longer, not pay much attention to sell by/use by dates. If products with an early sell by date are at the front, and later ones at the back, they will be sold first. If things are organised the other way round, or stock is improperly rotated, newer stock will be sold first, leaving out of date stock sitting on the shelves which will have to be thrown away.
Rotation also applies to loose products; in this case, there is usually no set sell by date, and produce must merely look fit to eat. Older stock is merely placed on top of newer stock to rotate it.
Some customers are fully aware of the practice of rotation, and will reach towards the back of the shelf in order to get newer (and therefore slightly better) produce. Also, when applied to large amounts of produce, rotation can be difficult if not impossible. It only takes one careless worker to disrupt rotation and create problems.
Other methods of getting rid of stock
If a stock is nearing its sell by date, stock may be reduced, that is its price lowered so as to sell off certain stock quicker. This is usually combined with rotation, so that reduced stock is at the front and so more noticeable to customers.