Basically, if a company decides that a website has something that violates their copyright for any reason, SOPA would allow them to shut down the website and put the owners of the site in jail. Regardless of whether the site's owner had any control over that content.
So when MacGuyver posts a picture of something he built based on a character from D&D, if the copyright owners of D&D decided that his doing so violated their copyright, RPGFan's owner (and staff? I'm not clear on that) would be held criminally liable, prosecuted, and potentially jailed for years.
Obviously, that is stupid. And as I understand it, it's not even remotely the goal of SOPA. They really just want to stop piracy, particularly from sites based in other countries. But they did a crappy job of writing the bill, so it's way, way too vague. That's how it sets up scenarios like my example above.