Gold karat is a measure of how much of the precious metal is contained in a piece made with it.
If a jewelry item is 18 karats, this simply means that out of a total of 24 parts, 18 parts of the piece are gold, and 6 parts are made up of other metals.
Expressed in percentage terms, 18 karats is equal to 75% gold content (18 divided by 24). The other 25% usually consists of metals such as zinc, copper, nickel, etc.
The additional metals in 18K gold serve to make the alloy harder and more durable than pure gold (24K), which is too soft for jewelry.