If someone wants to say something that could potentially spoil the plot: give away the ending, tell that an important character is going to die during the game, or reveal a major twist in the story, etc., they put "spoiler tags" around that information. The spoiler icon looks like this and is above the screen where you type your post. If you are going to post a "spoiler", please highlight the text you want to hide and click on that icon.

To see the hidden spoiler text on someone else's post, just hold the left mouse button and drag it across the part of the message that appears to be empty, like (SPOILER)this stuff right here, to highlight the hidden info; but you've been warned that it might ruin parts of the game if it's your first time playing it.