The door harp originated in Scandinavia, mostly in Sweden but also in Norway and Denmark and has evolved over time. It's estimated the door harp has been around for about 700 years. It was inspired by the Chinese Feng Shui tradition of hanging bells or chimes around doors and windows to "alter the Chi" (good energy in, bad energy out).
The Swedish adopted this idea but instead of bells, they incorporated the strings and beads which you see on the modern day door harp. The original superstition behind them was to repel evil spirits but has since evolved to signify welcome, luck, health and prosperity to all that pass through the door. It is usually hung on the inside of the front door (some hang it on the outside but this will make it easier for the wood to get warped and the strings to go out of tune)
The door harp is a flattened hollow box made from various kinds of hardwood that comes in many different shapes and sizes, with hanging balls that bounce off the strings which are tuned to different notes. The amount of strings can vary from 3 to as many as a dozens (or more!). Traditionally the strings were made of braided silk, however, these days they are usually wire.