Anything valued is very important — it's admired or treasured. If one of the most valued members of your softball team is out sick, everyone will miss her — and you risk losing the game.
The adjective valued comes from value, and it essentially means "considered to have value." Your valued possessions may literally be the most expensive items you own, or they may simply be things with sentimental value, like the photo of your grandparents or your dad's old watch. Marketers use this word all the time, often referring to valued customers or valued business.