Explain how the term “frontier” is Eurocentric.

Well, depending on where is applied, this term can represent European dominance over a people. Once the Old Continent (Europe) expanded its control over the world between the 16th and 19th centuries, several areas of the globe were under the European domain. So, they create and manage the frontiers of these countries and continents as they wish, and the Europen countries didn't use to respect the original frontiers of their domains.  

Explanation:

It's normal that every term referred the to European control over America, Africa, and Asia continents as "eurocentric" once until the 19th century these locations were under their control. Europe controlled these areas based on their beliefs and political ideas.