PPP takes a basket of goods and measures the price of them in different countries. Naturally the items in the basket of goods can shift the PPP measurement. Worldbank's methodology explanation is here: https://www.worldbank.org/en/programs/icp/methodology

Purchasing Power Parity is a measure of the country's economy in terms of their own currency, rather than relevant to the dollar as GDP is typically measured. The reason why'd you want to do this is to gain a more "objective" sense of the size of a country's economy, outside of the whims of tariffs and taxes and transportation costs and other things which may affect what, say, a loaf of bread may cost in one country compared to another. PPP calculations often use hundreds of representative items to form an accurate measure of PPP.

It's tempting to be like "Oh, so what's even the point of measuring GDP in dollar terms then? It isn't representative of the size of economies if China and the US switch places, and by several trillion dollars!" The reason why we can't unfortunately just ignore GDP without PPP is because the dollar is the largest international reserve currency, with many goods being priced in it and most countries using it in trade, so it is actually relevant what the "dollar's perspective" (if you will) of economies is.

Though, honestly, I'd recommend moving away from GDP entirely, PPP or otherwise. Its usage as a very general average of economic growth tends to mask a lot of other relevant effects - electricity growth, industrial growth, resource extraction, poverty rates, quality of life, life expectancy, etc etc etc, all behind a single figure designed to suggest "Well, big number go up therefore economy must be gooder!" I don't think it really has that much predictive power. From the perspective of GDP, for example, Russia lasting this long and even winning against the combined power of every Western country is totally inexplicable - Russia has something like a fiftieth of the combined GDP of the West. It was a common joke back at the start of the war that Russia has the GDP of Italy and yet wants to challenge all of us!

And this isn't even mentioning that GDP is very goofy measure, because it measures all economic transactions. This sounds okay in theory, until you realize that - yes, the absolutely massive healthcare bills that reach into the tens or hundreds of thousands of dollars for a patient having a single major accident/medical issue; those all count towards GDP. What is superior: a hypothetical country where healthcare was very efficient and effective, with only small costs for patients (ideally paid through taxes and thus "free" when needed by a patient) and very good standards of care? Or a country where healthcare is very inefficient and extremely expensive, where most people go into extreme debt just to pay for necessary surgeries? According to GDP, the latter is a much bigger, and thus better, economy. And this applies for all major, unnecessary costs in an economy, including incredibly overpriced rent.

Because GDP is so easy to inflate, if a country's GDP goes down, then you know something has critically fucked up, especially in Western countries which are increasingly financialized and where things are getting more expensive and property owners like Blackrock are forming monopolies and thus able to jack up prices with little consequence. Even stagnation or only small growth in a Western country is really bad. It's why the status of being technically in a recession (2 quarters of negative GDP growth) is becoming less relevant of a measure of economic hardship. America isn't technically in a recession and yet the economy fucking sucks for the 90%, thanks to inflation and other such effects, but liberals just see the big GDP number and are utterly befuddled why people think the economy is bad when it's actually great according to that number, coming up with all sorts of conspiracies about the media instead of, like, exiting their bubble of rich investment bankers or media analysts and talking to poor people.

PPP removes currency/pricing affects. Under normal GDP, a $10 aspirin pill at the hospital in the US adds $10 to US GDP. That same aspirin pill costs $0.10 in China and thus only adds $0.10 to Chinese GDP, even though both countries produced 1 aspirin pill. GDP PPP prices both pills at the same price to determine who is actually making more stuff.