People usually use the term for poorer countries, as opposed to the rich ones. Originally third world meant those not aligned with the USSR or NATO, I believe.

I get the meaning of "third world" but the USSR hasn't existed for decades, is China the modern equivalent? And it wasn't made clear why a neutral country like Switzerland would be expected to be highly corrupt while Russia would be low in corruption. Or indeed why Switzerland would be seen as a country in the process of becoming financially rich.