A clue as to the meaning of this piece of figurative language is the inclusion of the character 口 (kǒu). In nature we may see a hole like this as the entrance to a cave, or in our everyday lives as an extrance or exit. We can also call this hole or entrance the mouth of a cave. This is our first clue as to the meaning of the phrase 朗朗上口. We know that it is connected to our mouth. And what do we produce from our mouth? Speech, or sometimes song.
Our next clue are the characters that begin this item of figurative language. They are the character 朗 (lǎng) used twice. This actually means clear , or bright. Remember in English we also associate the word clear although often as an adverb: speak clearly.
However, in this usage, when we combine the characters 朗朗 and 口 it means that something is memorable, or catchy. So if we have heard a certain song once or twice we can easily produce the song ourselves. It rolls off our tongue. This is because the tune is catchy, or memorable
The third character in the phrase 朗朗上口 is 上. This means to come up, and here it suggests that it is easy for us to produce, So we have characters meaning: clear, clear, produce mouth. In other words catchy, as in a tune or song. Below is an example of this figurative phrase in use:
这首小儿歌呀， 还朗朗上口。我们可以边拍手读儿歌吧。
Zhè shǒu xiǎo ér gē ya huán lǎng lǎng shàng kǒu. Wǒ men kě yǐ biān pāi shǒu dú ér gē bā
This children’s song is very catchy. Let’s clap our hands as we read.