916 (22K) gold is about 91.6% pure, while 999 (24K) gold is about 99.9% pure. 999 gold is the softest, purest, and most malleable form of gold. This makes it an easy material to craft with, but scratches and dents easily.
To overcome this, 999 gold is often mixed with other metals. The term 916 is used to denote that the piece is made of 91.6% pure gold, with the remaining amount being composed of metals like copper, silver, and zinc.
For instance, if you have 10 grams of 916 solid gold, it would contain 9.16 grams of pure gold; and the rest of the weight would come from additional mixed metals.