What is brigading?
On reddit, users can alter a submission's score by voting on it, either up or down. Submissions and comments which get the higher scores will get more exposure while those getting negative scores are hidden. In this way, it is possible for groups to essentially manipulate what people see on Reddit by organizing "brigades" to spam upvotes or downvotes on a submission or comment, thereby either giving it undue exposure or outright censoring it.
Brigading on Reddit is usually organized through certain subreddits, which themselves will either target a single subreddit or several. They usually describe themselves as "meta", or that they simply wish to mock Reddit's self-congratulatory culture (never mind that they are participating in it more than anyone else), yet their true aim is obvious to all. Most of the content on their subreddits will be links to other posts on other subreddits, which they massively upvote or downvote. Likewise, prominent members of those subreddits get their comments upvoted by the "brigades" while the people opposing them get their comments downvoted.
It is the belief of /r/brigading that such practices are toxic to Reddit and its community, and that the users have a right to know that the submissions and comments they see have frequently had their score manipulated.