The basic point that is made by world systems theory (associated with Immanuel Wallerstein) is that we need to look at the whole world, rather than at individual countries, if we want to understand why some countries develop and others do not. This theory holds that there is an international division of labor that tends to allow some countries to develop while preventing others.
Wallerstein argues that the world can be divided up into a "core" and a "periphery." The core is made up of the countries of the rich world. These countries are technologically advanced and remain rich by producing goods and services that are expensive and require technology and/or high levels of skill to make. The periphery is made up of the less-developed countries. These countries remain relatively poor because their place in the division of labor is to do the unskilled, low-value work.
In modern times, the core has been Europe and its offshoot countries like the US and Australia. It has also included some Asian countries. The periphery has been the less-developed countries in Latin America and Africa in particular.