Wikipedia isn't blocked, they are voluntarily blacking out their website for 24 hours in protest. Lots of other sites are protesting as well like icanhascheezburger and The Oatmeal.
SOPA stands for Stop Online Piracy Act. Basically, the US doesn't have the power to shut down off shore pirating sites. So they are going after on shore sites that host pirated material. Under the current system, if a site hosts pirated material, they can simply remove it as the request of law enforcement or the owner. But under SOPA, the government has the power to shut down the website.
Simple, right? Just don't put pirated stuff on your website. But websites, like this one and Wikipedia, are edited daily by outside individuals (like me, writing this response). If I put a link for free stolen music or movies, Y!A becomes liable, not me. Sure they can ban me and remove the content, but the government can theoretically shut them down or charge them simply for hosting it. The biggest issue is this can all be done without due process and can not only affect the websites, but people's lively hoods if they use those sites to make money.