Commonly used as an abbreviated way to say "for real".

Usually used in texting, as many people use as many abbreviations as possible.
It can be a form of agreement towards someone, or disbelief.
If you use it as disbelief you may put a question mark to emphasize that you're asking if they're being serious.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.

"for real" (usually used online. And if used in real life by anyone, stay far away for them.)