9K gold is 9 karat gold, or 9 parts pure gold to 15 parts other metal. According to an excerpt from "How Products Are Made" hosted on Encyclopedia.com, the karat (K) is the unit gold alloys are measured in.
A karat, also spelled "carat," is equivalent to 1/24 part pure gold within an alloy. 24K gold is pure gold. The source states that gold is typically alloyed with another metal as a measure taken against the relative softness of gold. Alloying with silver, copper, platinum or nickel makes the gold harder and more suitable for use in industry. The industrial applications of gold are mainly in electrical, electronic and ceramics industries and make up around 25 percent of the gold market.