What does karat mean for gold?

Karat tells you how much of a piece is pure gold, out of 24 parts.

24K is almost pure gold, about 99.9%. 22K is 22 parts gold out of 24, or 91.6%. 21K is 87.5% and 18K is 75%. The rest is usually copper, silver or other metals, which make jewellery harder and less likely to bend.

A lower karat costs less per gram because it contains less gold. It says nothing about how well a piece is made.