.

Stops a sentence. It's end punctuation.

It's that simple. Or is it? Usually it's used (in text) nowadays to symbolize someone being angry or upset.
Or it's used in its form "." (in text)), which to be honest is kinda stupid. If you're not gonna say anything, don't say anything at all~.

Example 1:

(user one): *intense long rant*
(user 2) .

Example 2

(user one): Yo, you okay?
(user 2) Yeah. I'm fine.
(user one): You sure dude?
(user 2): Yes. -_-
(user one): i- okay-