It's basically used to describe caucasians, though African-Americans will sometimes also be known as farang or as farang dam('black farang'). Farang is also the Thai word for the guava fruit, so you can expect to hear farang eating farang 'jokes' if you happen to purchase any.
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I agree with you that farang meaning is a Thai fruit, but one my friend told me that farang also known as foreigner people. I'm just confuse, can you please explain this word deeply?