A Spoiler is a handy feature which will allow you to 'hide' information in a box that needs to be clicked on to open. It will allow to talk about a certain topic to specific people who wish to read what you say. Should someone not wish to read it, they don't have to but can still follow the general topic in the thread.
A common use of a spoiler is to enclose information about a TV episode that some people may not have seen yet, and may not want the surprise spoiled. Hence the name 'spoiler'.
Another common use of the spoiler function is in signatures. It is used to enclose a lot of personal progress with freebie sites to allow people to keep track of their accounts and not waste a lot of space on people's screens who are trying to read through a thread.
A spoiler looks like this:
To create a spoiler in this forum, use the following:
[spoiler] Your information goes here [/spoiler]