This is a preposition meaning “about” or “concerning”.
la þō jatāena xiēn jē anþēŋŋeni;
This story is about an argument.
The titles of many short texts consist of xiēn jē plus a noun phrase. For example, the title of the Kēleni version of the North Wind and the Sun is xiēn jē anþēŋŋeni “Concerning an argument”.