Meaning:

“弹” means “bullet,” and “幕” means “screen.” “Bullet screen” is a new feature on online video sites in China and Japan, which allows real-time comments from viewers to fly across the screen like bullets. Mostly used for virtual nods and zingers, this social viewing feature is phenomenally popular with the younger crowd. In fact, it is so popular that several theaters in China have been incorporating this practice into special screenings that display streams of text messages sent in by the audience. People also pay streaming sites such as Tencent to have their comments run on the screen of live broadcast of sports games.

Example:

A: 有时候在网上看电影真心受不了各种弹幕评论。

Yǒushíhòu zài wǎngshàng kàn diànyǐng zhēnxīn shòubuliǎo gèzhǒng dànmù pínglùn。

Sometimes I find it hard to put up with all types of weird bullet-screen comments while watching a movie online.

B: 你可以选择关掉弹幕功能的。

Nǐ kěyǐ xuǎnzé guāndiào dànmù gōngnéng de。

You can actually turn off the bullet-screen feature.