The word farang comes from the word for France, Farangseht. It seems the French were the first caucasians seen in Thailand which at the time was Siam. So now the word is used to refer to caucasians. If you visit Thailand you will be refered to as farang. Farang is also used in the words for potatoes and chewing gum. Farangs in bars are caucasian tourists or expats, generally male, who spend time in bars socializing and enjoying entertainment which Thailand it reknowned for. The most famous places are districts in Bangkok such as Soi Cowboy, Nana Plaza and Patpong and Pattaya which is 2 hours southeast of Bangkok. I hope this answers your question.
farang means foreigner of white origin or in other words, westerner/ caucasian