The two characters combine literally to mean “mouth sound,” which perfectly describes the concept of an accent—the specific sounds that come from a person's mouth when they speak.
In China, 口音 is a much more prominent and discussed topic than in many Western countries. This is due to the vast geography and the relationship between the official language, Standard Mandarin (普通话, pǔtōnghuà), and hundreds of local dialects (方言, fāngyán). Almost every Chinese person who grows up outside of Beijing speaks Mandarin with a 口音 influenced by their native dialect. This accent is a powerful signifier of identity and origin. A trained ear can often tell if someone is from Shanghai, Sichuan, Dongbei (the Northeast), or Guangdong just by listening to them speak for a few seconds. This can be compared to regional accents in the United States, like a Southern drawl or a Boston accent. However, the linguistic distance is often greater in China. The difference between a Sichuanese accent and a standard Beijing accent is often more pronounced than the difference between a Texas accent and a California accent because the underlying dialects (Sichuanese vs. Mandarin) are significantly different. Historically, the accent of Beijing and the Northeast was considered the most “standard.” Today, while news anchors and teachers strive for perfect 普通话, there's a growing appreciation for regional accents as a form of cultural diversity. However, having a very “heavy” (重, zhòng) accent can sometimes be stereotyped or perceived as less sophisticated in formal business or academic settings.