Okay, so imagine you have a bunch of different types of underwear for boys and men. Some of them are really fancy and cost a lot of money, and some of them are cheaper and more basic.
The men's underwear index is a way of figuring out how well the economy (or the way people in a country are doing financially) is doing by looking at what kind of underwear guys are buying.
If guys are buying fancier, more expensive underwear, it could mean that they have more money to spend and so the economy is doing pretty well. But if guys are buying cheaper, more basic underwear, it could mean that they don't have as much money to spend and things might not be going so great for the economy.
It's kind of a silly way of looking at things, but economists who study money and how it affects people like to use all sorts of weird things to figure out what's going on!