The word "bulk" is put there by your email provider. I see this on accounts provided by yahoo, so I guess it's them doing it. I'm not sure what criteria they use to identify "bulk" mail. Usually it's what I'd consider to be "spam" or more formally, UCE ("unsolicited commercial e-mail"). As such, unsubscribing from it would most likely be a bad idea as doing so simply confirms your email address as being valid and active, which make it valuable to spammers.

At a slightly different level, you could set Thunderbird not to download this folder, that is, unsubscribe from the folder. But I can't recommend doing this, as like pretty much all spam detection systems, it can make mistakes and you may occasionally have good messages mistakenly placed in the bulk folder.