The word fashion has to do with customs and trends. Popular clothes are in fashion. And someone who keeps up with the latest styles is a follower of fashion.
The most common use of fashion is to describe the hippest, trendiest clothes and music — we say popular things are in fashion. But this word can also refer to a way something is done, as in "We train dogs in a certain fashion." To fashion something means to built it, as in "We fashioned a shelter out of rocks and mud." But that kind of fashioning probably isn't in fashion these days.