A flash freezer is a freezer that is able to rapidly freeze items by exposing them to extremely low temperatures, typically well below zero degrees Fahrenheit. These are most often used to allow perishable food items to stay fresh longer while also retaining as many nutrients as possible.
The reason flash freezing is beneficial is because the speed at which a perishable item freezes has a large effect on the nucleation process and the size of the ice crystals within the item. The high heat removal rate of the freezer allows smaller, more ubiquitous ice crystals to form and also increases the nucleation rate. This in turn causes less damage to the cell membranes of the item frozen, and allows it to stay fresh much longer than using conventional freezing methods.