The app measures a difficulty of a given text written in Chinese Mandarin. The app groups characters by their HSK levels.

The Hànyǔ Shuǐpíng Kǎoshì (HSK) (Chinese: 汉语水平考试), translated as Chinese Proficiency Test, or Chinese Standard Exam, is China's only standardized test of Standard Chinese language proficiency for non-native speakers such as foreign students and overseas Chinese.

It is not uncommon to simply refer to a standard or level of proficiency by the HSK level number, or "score." For example, a job description might ask for foreign applicants with "HSK5 or better."