The item you are referring to is commonly known as an ice pack or gel pack. Ice packs are designed to absorb heat and maintain a lower temperature when placed in a freezer. The contents of an ice pack typically consist of a gel-like substance that remains flexible even when frozen. This gel is typically made from water mixed with chemicals like propylene glycol or sodium polyacrylate.
When you place an ice pack in the freezer, the gel inside cools down and freezes, turning into a solid. The frozen ice pack can then be removed from the freezer and used to keep perishable items or beverages cool in coolers, lunch boxes, or other situations where refrigeration is not available. As the ice pack is exposed to higher temperatures outside the freezer, it gradually absorbs heat from its surroundings and melts back into a gel, maintaining a cooler temperature for a longer period than if you had used ice alone.
Ice packs are commonly used for transporting food, preserving medication, relieving pain or swelling, or keeping beverages cold without the mess of melting ice. They are reusable and can be refrozen multiple times, making them a convenient cooling solution.