They're for adding an emotion to your text. Mostly just for fun.

[The Underground was full of 'em. Mostly thanks to Alphys being Alphys. He kind of misses Alphys.]

Bottom row's, uh...salt's for if you're upset, I think. The gun's for when you're real angry, and so's the skull. And the third one in is...

[Cripes, really? This is a kid.]

It's kind of a rude human gesture. For five-fingered folk.