I am not going to say anything better than what the major sites like Google or Wikipedia are going to say, so here are the links to their pages explaining it.
Remember that in the USA, we are supposed to be the land of the Free, respecting an individual's rights, which are not to be taken away without Due Process. The protest is not about allowing piracy; it is about ensuring that everyone is given Due Process in determining wrongdoing.
Also, many other sites explain it even better.
http://americancensorship.org/infographic.htmlOn Wednesday Jan. 18th thousands of sites will go dark to protest SOPA & PIPA, two US bills racing through Congress that threaten prosperity, online security, and freedom of expression.