It makes something last longer. For example, jam keeps for a long timewithout going off because it has a lot of preservatives in it. Sugarand salt are common preservatives...www.flashpapers.com
It means a chemical or other process added to a food or another ingredient to keep it from spoiling or rotting (to preserve its freshness).
These are chemicals that are added to food to make them last longer. Mind you some perservatives happen naturally in the fermentation process such as beer and wine that is why the older the wine, the better