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In the context of Reddit, (A online community involving a myriad of "subreddit" forums called "subs", each with their own distinct culture and moderation) a containment sub is a sub that is usually more lax in some area of moderation. This is to let a certain subsect of people (that are usually running counterpoint to Reddits main company, particularly in political issues) congregate there and not spread into other subs and "infect" them with their ideology. Certain topics (such as articles about the ponzi scheme Ellen Pao's (the CEO of Reddit) husband created, can cause you to be shadowbanned in many forums, yet in a containment sub they generally will stay viewable indefinitely.

One particularly notable example of these is /r/kotakuinaction, which deals with the Gamergate movement. (and in particular is anti-SJW, whereas Reddit as a company is more definitely pro)
Don't worry about posting that, we're in a containment sub, your account is safe from the mods.
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