"萌 (méng)" literarily means to sprout and is the symbol of spring. From this, the meaning of "萌 (méng)" has been extended to describe girls who are young and innocent. The term originated from the animation industry and was used as a lingua franca to refer to young, innocent and beautiful little girls.
Another popular phrase is "卖萌 (màiméng)" which is a verb and means to "pretend to be cute" to attract the audience. The word "卖 (mài)" literally means to sell, but here it is the short form of "卖弄 (màīnong)" meaning to show off.
Examples
Tā zài zhège diànyǐng lǐ yǎn de juésè hǎo méng a, wǒmen dōu hěn xǐhuan.
1. 他 在 这个 电影 里 演 的 角色 好 萌 啊，我们 都 很 喜欢！
The role he played in the movie was really young and innocent; we all liked it!
Mèimei zǒngshì zài wǒ miànqián màiméng hǎo ràng wǒ gěi tā mǎi táng chī!
2. 妹妹 总是 在 我 面前 卖萌 好 让 我 给 她 买 糖 吃！
My little sister always pretends to be nnocent in front of me in order to get candy from me!
Wǔ suì de nǚ'ér chuānzhe fěnsè de qúnzi, yí shàng tái jiù méng fān quánchǎng.
3. 五 岁 的 女儿 穿着 粉色 的 裙子，一 上 台 就 萌 翻 全场。
