Learn German: Ätherisches Öl ist oft teuer. - Essential oil is often expensive.

It describes the neuter noun Öl (das Öl) in the nominative case. Because there is no article like das or ein in front of it to show the gender, the adjective has to do that job. This is called strong declension, and for neuter nouns in the nominative case, we add the ending -es.

If ätherisches gets an ending, why doesn't teuer get one too?

Adjectives in German only get endings when they come directly before the noun they describe (like ätherisches Öl). When an adjective comes after a verb like ist to describe the subject, it stays in its base form. Therefore, teuer takes no ending here.

Ätherisches looks like the English word "ethereal". Is that where it comes from?

Yes, they share the same root! While it literally means "ethereal," the phrase ätherisches Öl is the standard German term for "essential oil." Also, as a quick pronunciation tip: in German, a th is always pronounced exactly like a regular t.