A spoiler is a picture or a text that spoils an episode, comic or film by telling some themes or big reveales on forehand. For example, explaining on forehand that Darth Vader is Luke Skywalker's father would be to spoil the second Star Wars film.
The Infosphere's policy on spoilers is simply that they are everywhere. By entering the Infosphere, you are immediately expected to either have seen all episodes, or to have them spoiled and deal with it. An article can always show in the introduction that a character is dead, that someone is someone else's father or how the series (eventually) end.
If you don't like spoilers (most people don't) and you haven't seen the latest episode just yet, the best recommendation is to stay off the site until you have.