No, you don't understand! Those people that leave their homes and in some cases, their families behind, to go on a mortally dangerous journey without a garantee to find better lives - they just want to take your precious culture away from you! /s
It literally translates to "end sarcasm." The slash is a coding symbol in HTML (and possibly other languages) commonly used to indicate an end or a stop of something. For instance if I wanted to bold something I would say <b> and then </b>.
I assume reddit helped it evolve although I have seen it around other forums. It would take a programmer, probably, or a math person (/= sometimes is used to mean "does not equal") to be familiar with the use of the slash as such. And I'm sure it started as /sarcasm and went from there.