Meaning:
This mysterious code is actually the initials of Chinese pinyin “yǒu yī shuō yī” (有一说一), which is literally translated as “when I have one, I’ll speak one.” This Chinese saying, often added in front of a sentence, means “to be honest.” A lot of netizens use it in bullet screen comments on streaming sites or in other online conversations.
Example:
A: 马路对面新开的饭馆你去吃过吗？明天有个客户来拜访，我打算请他吃饭。
Mǎlù duìmiàn xīnkāi de fànguǎn nǐ qù chī guò ma？Míngtiān yǒu ge kèhù lái bàifǎng，wǒ dǎsuàn qǐng tā chīfàn。
Have you tried out the new restaurant across the road? A client of mine is visiting tomorrow, and I plan to treat him to lunch.
B: U1S1，这家饭馆做的饭菜味道还不错，不过是普通家常菜，不太适合请客。
Yǒu yī shuō yī，zhèjiā fànguǎn zuò de fàncài wèidào hái búcuò ，búguò shì pǔtōng jiāchángcài，bú tài shìhé qǐngkè。
To be honest, the dishes they prepare are delicious, but are not suitable to treat guests, because they are common home cooked style dishes.