Firstly, what actually is a spoiler? It’s where a plot detail in a film, book, TV show or whatever, is revealed to people before they are able to see the film/show, read the book, etc for themselves. Sometimes a spoiler can be fairly mild, revealing nothing more than the name of a new character, but they can also be very severe, revealing a significant plot point, and spoiling the viewer’s enjoyment if they know what’s coming (hence “spoiler”).
So, then, what’s spoiler space? Well, a long standing convention on Usenet is to employ “spoiler space” when revealing plot details. This is a mutually acceptable approach between those who wish to discuss what they’ve seen (for whom such a discussion isn’t spoiling anything), and those who don’t wish to discuss what they haven’t seen (for whom such a discussion is spoiling things). Spoiler space is simply a couple of dozen blank lines inserted before any comments that include spoilers (with a note above those lines to say what film, TV show, etc is being spoiled). The idea is that people see that bit above the blank lines telling them what is potentially being spoiled below, and decide for themselves whether they want to scroll down and read/participate in that discussion. (The point of the blank lines is to force the actual spoilers down the screen, hopefully off the bottom, so that they aren’t read by accident – good peripheral vision can be a dangerous thing!)