Knowing that "un-" means "not," you can probably figure out that someone who is unappreciated is someone whose value is not recognized or rewarded. We all feel unappreciated at times, but feeling that way might help us appreciate others.
The adjective unappreciated took shape in the mid-19th century. It evolved from the Latin appretiāre, "to set a value on something." The word unappreciated can refer to anything that does not receive full credit, either positive or negative. For example, we might say that the possibilities of disaster due to global warming are still unappreciated by most people.