hreflang is markup that tells search engines which language or region each page is for when the same content exists in several language or regional versions.
You mark Korean pages with ko, English pages with en, Japanese pages with ja and Traditional Chinese pages for Taiwan with zh-TW, and link them to each other. The page to show by default is marked x-default.
Without hreflang, languages can get crossed, for example a Korean page shown to customers in Japan.
