Mostly, it can mention a kind of music, clothing and culture etc , and usually, we consider "indie rock" to be a kind of 非主流音樂.

In my opinion, generally ＂非主流＂means sth that not predominant in a society, not well accepted by people. It can be referred to culture, religion, opinion and so on. But nowadays in china, 非主流 is often used to describe a certain group of people, they like to be special, they are proud of being different from most of the people, for example they dress weirdly or have a strange hairstyle, even create their own language, I mean some strange phrases or expressions. Sometimes they don't care so much about others. you can say 这个人很非主流 or 非主流发型 or 非主流的everything if you like :)

非主流 means NOT 主流. it means some people who do something in some way that most people never do. for example, normally we color our hair with black, yellow, brown etc. this is 主流. but some guys color their hair with green, so they are 非主流. normally, 非主流 in china is a negative word. it means those guys whose hair is weird, whose clothes are weird, whose behaviour is weird etc.... here is an example: http://image.baidu.com/i?ct=503316480&z=0&tn=baiduimagedetail&ipn=d&word=%E9%9D%9E%E4%B8%BB%E6%B5%81&step_word=%E9%9D%9E%E4%B8%BB%E6%B5%81&pn=7&spn=0&di=67290951970&rn=1&is=&istype=&ie=utf-8&oe=utf-8&in=31716&cl=2&lm=-1&st=&cs=1532877880%2C1782464161&os=3606736520%2C3511502190&ln=1000&fr=&fmq=1409861548140_R&ic=&s=&se=&sme=0&tab=&width=&height=&face=&ist=&jit=&cg=&objurl=http%3A%2F%2Fimg1.qq.com%2Fcq%2Fpics%2F15131%2F15131851.jpg&fromurl=ippr_z2C%24qAzdH3FAzdH3Fvq_z%26e3Bqq_z%26e3Bv54AzdH3FwAzdH3Fdaab8dnaAzdH3Faaaa9c_z%26e3Bip4

It means “non mainstream”, means something only some people know it or like it, and not very popular among the world.

非主流 means not popular, not every one like it