Thanasi, you have understood perfectly well what brain drain is! Here is the defintion we talked about (Wikipedia):
Human capital flight, more commonly referred to as "brain drain", is the large-scale emigration of a large group of individuals with technical skills or knowledge. The reasons usually include two aspects which respectively come from countries and individuals. In terms of countries, the reasons may be social environment (in source countries: lack of opportunities, political instability, economic depression, health risks; in host countries: rich opportunities, political stability and freedom, developed economy, better living conditions). In terms of individual reasons, there are family influence (overseas relatives, and personal preference: preference for exploring, ambition for an improved career, etc). Although the term originally referred to technology workers leaving a nation, the meaning has broadened into: "the departure of educated or professional people from one country, economic sector, or field for another, usually for better pay or living conditions".Brain drain is usually regarded as an economic cost, since emigrants usually take with them the fraction of value of their training sponsored by the government or other organizations. It is a parallel of capital flight, which refers to the same movement of financial capital. Brain drain is often associated with de-skilling of emigrants in their country of destination, while their country of emigration experiences the draining of skilled individuals.
Also, do you remember the words 'upstream' and 'downstream' mentioned in one of the texts of the book? They were connected to the computer science in the specific context, so here are two convincing explanations: