The nyckelharpa is a folk instrument from Sweden
with a long standing tradition there. "Nyckelharpa" is a Swedish word, "nyckel" meaning "key" and "harpa" being a general term for a stringed instrument. Its key features (hah, see what I did there?) are the sympathetic strings and a keybox full of wooden keys. Some translate it into English to "keyed fiddle"
"viola de teclas" in Spanish or "viola d'amore a chiavi" in Italian - although it is not a fiddle or viola ;)
How it works: I produce the sound with a bow on the strings, like on any other bowed instrument, but instead of touching the strings with the fingers of my left hand and pushing them down on a fretboard or fingerboard to play a melody, I press a key. The key moves a little wooden tangent to the string and tada, the note changes
The magical sound of the nyckelharpa is created from its sympathetic strings (usually 12) which are enhancing every note I play on the four melody strings.
If you want to hear some nyckelharpa music, you're welcome on my duos bandcamp https://aklejafolk.bandcamp.com or search for "Akleja" wherever you like to stream music. More names you can look for there are Josefina Paulson, Hazelius Hedin, Emilia Amper, Väsen, Daniel Pettersson / Sotali, Erik Rydvall - there are so many FANTASTIC players out there! 
The instrument pictured here was made by Mats Wester.