The tag is normally [spoiler][/spoiler] but doesn't appear to be available on this forum (not sure how easy it'd be to add here as the other forum I'm on that has it is also powered by phpBB.
Here's a link to a post of mine on a minecraft forum that uses the tag. The image hidden by the tag is broken (was having some trouble with dropbox):
http://honeygoes.freeforums.org/post3069.html#p3069 Please remove the link if I've inadvertently broken a linking rule here.
The tag is normally used to hide spoiler type information. for example, providing hints to a puzzle in a game as well as the solution, but by putting the solution behind a spoiler tag people who only want a little push in the right direction rather then being given the answer can avoid accidentally ruining it. It could also be used on movie review forums.
Here it's more suited to hiding sets of images so when revisiting a thread readers don't have to scroll as far to get to the newer stuff (as for me, using the 'new posts' link will load the page and show the new post, but then all the images in the thread load pushing the new post down and I have to scroll through all the prior posts and hope I can identify where the new posts start). A variant tag is the [nsfw][/nsfw] which is (hopefully obviously) used to hide content that is 'not safe for work'.