If something is described as obscure, it is:

When something is described as obscure, it indicates that it is difficult to understand or perceive clearly. The term often refers to ideas, concepts, or pieces of information that are not well-known or are vague, making them challenging for individuals to grasp fully. This aligns with the definition of the word "obscure," which suggests a lack of clarity or visibility.

In contrast, terms like "clearly comprehensible," "brightly illuminated," and "commonly known" imply clarity, brightness, and familiarity, which are opposite in meaning to obscure. Thus, by identifying "difficult to understand" as the correct answer, one accurately captures the essence of the word, enhancing comprehension of its usage.