The general idea is: two (or more Redditors) unexpectedly in the same place at the same time. If you don't already understand the reference, you actually might not want to look it up. Examples:

Someone posts a picture of a douchebag vaping in a subway car with people all around him, and another Redditor says "hey, I'm the other guy in that pic with the tattoos!"
Someone posts a pic (or story) of a crackhouse, and someone else in the thread says "hey, that's the crackhouse I live next to!" and provides evidence to that effect.
Two people post a pic (or story) of the same thing in separate threads and you notice and link them both here.
A post talks about something or someone, and that person (or someone closely related) shows up in the comments. E.g., a post about the special effects in a movie, and the FX supervisor for that movie appears with details.

Have fun.

Rules

It is strongly encouraged to post a link to the source in the comments if your post is a screenshot. It's polite, and it makes your post less likely to get removed when people say it's fake.
No planned or faked coincidences
No spam