English for specific purposes is when people learn the English language for a very particular reason. For example, a doctor who wants to treat patients in an English-speaking country might need to know a very specific kind of English that relates to medicine. Or a scientist who wants to publish research papers in journals that are in English might need a different kind of English that is specific to science.
So instead of just learning general English like you might in school, people who are learning English for specific purposes focus on the vocabulary, grammar, and other things that are important for their specific field. This helps them communicate better and do their job or study more effectively.