Questions & Answers about Mi az ellenőrzés célja?
Az is the definite article, meaning the.
In Hungarian, questions of the type What is X? are often built as:
Mi + article + noun phrase?
So:
Mi az ellenőrzés célja?
literally: What is the inspection’s purpose?
The article appears because ellenőrzés célja is a definite noun phrase: the purpose of the inspection/check.
Also, Hungarian uses az instead of a before a vowel sound. Since ellenőrzés begins with e, the correct article is az.