copied from the learn more page:

What exactly is Wikipedia doing?

Wikipedia is protesting against SOPA and PIPA by blacking out the English Wikipedia for 24 hours, beginning at midnight January 18, Eastern Time. Readers who come to English Wikipedia during the blackout will not be able to read the encyclopedia: instead, they will see messages intended to raise awareness about SOPA and PIPA, and encouraging them to share their views with their elected representatives, and via social media.

What are SOPA and PIPA?

SOPA and PIPA represent two bills in the United States House of Representatives and the United States Senate respectively. SOPA is short for the “Stop Online Piracy Act,” and PIPA is an acronym for the “Protect IP Act.” (“IP” stands for “intellectual property.”) In short, these bills are efforts to stop copyright infringement committed by foreign web sites, but, in our opinion, they do so in a way that actually infringes free expression while harming the Internet. Detailed information about these bills can be found here and here. You can also follow them through the legislative process here andhere. The EFF has summarized why these bills are simply unacceptable in a world that values an open, secure, and free Internet.