Something that's preserved is kept in its original state or kept alive and fresh. Preserved artifacts are protected from damage, often in a museum where they can be kept safe.
Preserved food is treated in some way that makes it spoil much more slowly than it would otherwise. Sometimes this is done by adding chemical preservatives, and other times it's a more natural process, involving drying, pickling, salting, or canning. Preserved fruit, cooked with sugar and sealed in jars, is also called preserves. These words all stem from the Medieval Latin preservare, "keep safe."