A Redditor is a person who uses Reddit, a social media app, and believes every meme is from Reddit. They also consider themselves superior, are emojiphobic, love star wars/anime, Keanu Reeves and Minecraft. They refer to themselves as dank because they use Reddit, and call those who don't normies.
A Redditard is an alternate name to the aforementioned person.

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.