The Third World is a phrase used denote the nations that were not aligned with either the United States (First World) or the Soviet Union (Second World), listed in order of development. Today, it is used simply to denote countries that are not very well developed, such as North Korea or South Sudan.
More recently, the term "developing world" has generally been preferred by some. Many people now break countries up into more new categories, such as economically-developed countries (EDCs), newly-industrializing countries (NICs), lesser-developed countries (LDCs), and least-developed countries (LLDCs).
If you are reading this, you are probably fortunate enough to live in a newly developed country or better.