The document defines brain drain as the emigration of skilled professionals from their home countries to more developed nations due to a lack of opportunities. It then provides vocabulary on English prefixes such as "un-", "in-", "ir-", "im-", "dis-", "re-", "under-", "over-", "pre-", "post-", "pro-", "anti-", "intra-", "extra-", "inter-", "super-", and "hyper-" and examples of how they modify the meanings of words.