Hidden things can't be seen — they're concealed in some way. A hidden video camera inside a teddy bear might secretly be keeping an eye on a new babysitter.
People with hidden feelings don't let their sadness, anger, and joy show; and hidden clues in a treasure hunt can't be easily found. Hidden things can be obscured deliberately (like hidden eggs at an Easter egg hunt), or simply out of view or undetected by you. Hidden is the past participle of hide, from the Old English hydan, which means "to hide or conceal," but also "to bury a corpse."