I'll try to give a summary, but it's pretty in depth. In Podesta's emails, posted to wikileaks, there's strange conversations where food is brought up in weird ways. Someone (possibly with an FBI background) on reddit posted that it sounded like code words a pedophile ring would use.
The reddit community used the code words along with Podesta's contacts and found one of his close friends owns Comet Ping Pong Pizza. This establishment had some inappropriate artwork for a family establishment, and a really creepy video surfaced of a show that was held there.
Then the reddit community and a forum called 4chan started digging into the social media of the people that were associated with Comet Ping Pong and the lady that does "spirit cooking", and they found really creepy stuff in those people's profiles. Then the ball really got rolling with redditors digging and connecting dots.