Karats are often mistaken to be a weight measure used by jewelers. Instead karats are a measure of gold's purity.
Pure gold is too maleable to be used as-is by jewelers: a ring made of pure gold would bend and loose its shape and be impossible to wear. So jewelers "dillute" gold by mixing it with other stronger metals.
One Karat measures the fineness of gold in 1/24 part which is 4.2%. Hence a 18 karat gold ring is made of 18/24 parts of gold, or 75% gold and 25% other metals.
Metals added to gold are of cheaper value, so for equals weight, the price of a jewelery item goes up when the karat count goes up too.