Imagine you buy a Laptop in China for $500. Your friend buys the exact same laptop in America but has to pay $1000. Now according to nominal GDP, your friend simply has a laptop that is twice as valuable as yours.
The idea of PPP and why it is arguably a better indication of how big an economy actually is, is that it attempts to tackle the problem of price difference for the same stuff. Basically it does what you do to calculate inflation. You create baskets of goods and services (service costs are usually the biggest difference) and compare the price for that by country. Problem obviously being finding comparable baskets. But it's still way more representative than nominal GDP. You can imagine that GDP in the the US, where services are about 80% of GDP, is overvalued compared to a country like India or China, where you get most of those same services for a fraction of the price.
That is why countries like India and China are way, way undervalued in the nominal GDP calculations that end up completely misrepresenting the real world economic powers that they have.