The characters combine to create a wonderfully vivid image: words that are so clear and resonant (`朗朗`) that they effortlessly go “onto the mouth” (`上口`) to be spoken. It's a perfect description of language that is a pleasure to say aloud.
In Chinese culture, there's a deep-rooted appreciation for the aesthetics of language. For centuries, education involved the memorization and recitation of vast amounts of classical poetry and prose. A text that was lǎnglǎng shàngkǒu was not only considered well-crafted but was also easier for scholars and students to learn and pass down through generations. The sound and rhythm of the words were often seen as just as important as their literal meaning. This can be compared to the Western appreciation for “catchy” advertising jingles, but its scope in Chinese is much broader. While a Westerner might call a McDonald's jingle “catchy,” a Chinese person could use lǎnglǎng shàngkǒu to praise a classic Tang dynasty poem, a powerful political slogan, or even a well-written paragraph in a business report. It reflects a cultural value for eloquence, rhythm, and the memorability of spoken language in all facets of life, from high art to commerce.
This is a very common and useful idiom that you will hear in many contexts. It is almost always used as a compliment.
The term is generally neutral in formality and can be used in casual conversation as well as in more formal written reviews.