The word "development" in the field of development studies refers to the development of countries that were once described as "Third World." In modern terminology, those countries are referred to as "developing countries." Development studies is concerned with studying issues that are particularly relevant to that type of country.
To see what sorts of issues this might include, you could look at the current issue of an academic journal in this field (the sagepub.com link below). You can see there are articles touching generally on the issue of how to promote development in developing countries without ignoring issues of social justice. This is a common theme in development studies because countries often pursue economic development without regard to social justice.
There are also articles about such things as the role of religion in developing nations and whether genetically modified crops are good for sub-Saharan Africa.
Overall, then, we can see that development studies covers a wide variety of issues. Anything that is particularly relevant to "developing nations" is a subject of study for someone in development studies.