Purchasing Power Parity (PPP) is measured by finding the values (in USD) of a basket of consumer goods that are present in each country (such as pineapple juice, pencils, etc.). If that basket costs $100 in the US and $200 in the United Kingdom, then the purchasing power parity exchange rate is 1:2.
For example, suppose that Japan has a higher GDP per capita ($18) than the US ($16). That means that Japanese on average make $2 more than normal Americans. However, they are not necessarily richer. Suppose that one gallon of orange juice costs $6 in Japan and only $2 in the US; then $6 in Japan exchanges to only $2 worth of US goods, since the Japanese can only buy 3 gallons while the Americans can buy 8 gallons. We have decided to use 1 gallon of orange juice as a reference basket of goods. Therefore we can create a PPP index for Japan in terms of the US as 1/3. Therefore, in terms of orange juice, the Americans are richer, and in this example the US has a GDP (PPP) of $16, unchanged since it is the reference currency. Japan, however, has GDP (PPP) of only $6 since the $18 in Japan can only buy 3 gallons of orange juice, which represents only $6 of US goods.