I'm not feeling creative enough to do a true ELI5, but I'll explain as best I understand.

We can look up how much currencies are worth of eachother. But that doesn't tell us what they're worth in the economy. As I write this I know that 1 USD (American Dollar) is worth 1.33 CAD (Canadian Dollar). But that doesn't mean that 1USD = 1CAD, just that we can exchange at that. To put it another way, if I can buy some good for 1 USD in America, that doesn't mean that I can buy the same amount of the same good for 1.33 CAD in Canada. This is because speculation and demand for currency rules this conversion.

So economists are looking for measures, or indexes, on how to compare things. You run into issues doing this because not all cultures value the same things equally. For example lets say you want to compare the cost of how much a loaf of bread costs across cultures. That works fine, until you run into a culture that doesn't use much bread and thus bread is considered a foreign food. Or maybe they don't have cheap mass produced bread you see in your typical Western culture. It doesn't work well there.

Enter the Big Mac. It's sold in an obscene amount of cultures, and is mostly the same in all. If a Big Mac costs $6 in America and $9 in Canada, you can say that "$6 USD has the same amount of purchasing power as $9 CAD."