SOPA and PIPA are a pair of bills pushed by the entertainment industry (MPAA, RIAA, and so on) to combat copyright infringement by throwing out the baby with the bathwater. Basically it takes the stance that having a free and open Internet is a bad thing because people use it to copy movies and things without paying for them, so let's not have a free and open Internet and maybe people will do less of that stuff.
In November, Facebook, Google, eBay, Zynga, AOL, LinkedIn, Yahoo, Mozilla (Firefox), and twitter jointly placed a full page add in the NY Times opposing SOPA and PIPA. Sadly, the media content industry has a lot of money and lobbying power, and most members of Congress don't understand the net.* Some people who understand the net very well wrote this Open Letter From Internet Engineers to the U.S. Congress in December. Signatories to that letter include almost everyone I can think of who played a prominent early role in creating the Internet and developing its key technical standards, including a lot of leaders of the Internet Engineering Task Force which oversees those technical standards today. And last week, the MIT Media Lab published their statement of opposition to SOPA & PIPA where they highlight censorship and human rights concerns.
So get in ahead of the rush. With Wikipedia (along with reddit, boingboing, craigslist, and others) protesting, Congressional phone lines will be swamped tomorrow. If you're in the US, call your Representative and both Senators today. You can look up their contact information by plugging your ZIP code into Congress.org.
You can learn a lot about the bills from the links above, but what you say on the phone call can be as simple as this:
When calling your Representative: "Please oppose the Stop Online Piracy Act if it comes out of committee."
When calling each Senator: "I hear that a vote on the Protect IP act is scheduled for later this month. Please vote against it."
* BTW, wanna help someone awesome who does understand the net get elected to Congress? Darcy Burner, Computer Science degree from Harvard '96, worked at several software companies. I've met her. She's both nerdy and awesome.