1(of a contagious disease etc) to spread; to propagate
2(of a style of clothing, song etc) popular; fashionable
Examples
wǎng qiú网球zài在xué sheng学生zhōng中fēi cháng非常liú xíng流行。
网球在学生中非常流行。
Tennis is very popular among students.
我想要听流行音乐。
I'd like to listen to pop music.
现在长裙不流行了。
Long skirts are out of fashion now.