Its been outdated since 1991, its a relic of the Cold War.
1st world countries: US and western allies, countries with stable democracies and the rule of law.
2nd world countries: USSR, China, basically the communist world.
3rd world countries: less developed countries which weren’t formally aligned with/a part of either of the previous two groups.
I get the phrases have evolved slightly since then, but the whole model isn’t really fit for purpose. There isn’t really a second world any more but we still talk about first and third world countries, might as well do away with the phrases entirely.