The joke is a play on words.  An 'era' is a long and distinct period of history with a particular feature or characteristic. 'Era' sounds like 'ear' which would have made the phrase 'in one ear, out the other', and generally means ignored.  In terms of fashion, an era of a fashion trend would mean it is currently 'in' or popular for one style, but once the era ends that style would be 'out' or unpopular in comparison to other modern styles.
Fashion is something that goes in one era and out the other .