It's a modified BBCode spoiler tag. Normally, for an unnamed spoiler, you simply use {spoiler}. If you want to name the spoiler, you add a "=NAME" within the tag, so it ends up looking like this: {spoiler=NAME}. The ending tag is the usual /spoiler tag though, no need to name it as well. (replace {} with [] obviously)
Quote to get BBCode: show Quote this post, and you will see the BBCode for this named spoiler