This phrase is usually used to ask about someone's field of study in a school or university. For example, you might ask this question to a classmate to find out what they are majoring in.
Here are some English translations that capture the meaning more naturally:
* What's your major?
* What are you studying?
* What program are you in?
* What field are you specializing in?
The most appropriate translation will depend on the context and the level of formality.