This refers to gold jewellery.
Pure gold is 24 carats or 1000 parts gold and anything other than 24 carats is an alloy (mixture of metals)
9 carat jewellery is an alloy and contains 375 parts...
Pure gold is 24 karats (24K). 22K gold is almost, but not quite, pure. Some expensive jewelry, especially outside of the US, is composed of 22K gold. Additionally, American Gold Eagle bullion coins...
24 karat gold is as pure as you can get it. You will find almost no jewellery that is 24 karat as it is too soft. 9 karat gold is 37.5% gold and 62.5% other metals.
24 carat jewellery is pure gold whilst 9 carat jewelery is not. It is still valuable just contains less gold. The advantage to 9ct jewelery is that it would be harder wearing.