According to the Wikipedia definition of Low-key lighting is,
accentuates the contours of an object by throwing areas into shade while a fill light or reflector may illuminate the shadow areas to control contrast. The relative strength of key-to-fill, known as the lighting ratio, can be measured using a light meter. Low key lighting has a higher lighting ratio, e.g. 8:1, than high key lighting, which can approach 1:1.
Low-key lighting often requires a bit more thought to set up properly and execute and isn't as easy as some other forms of photography since most shots can't be done in ambient light. I think it's one of the more challenging forms to learn and master but also one of the most rewarding.