SOPA (Stop Online Piracy Act) allows copyright holders to force ISPs to block websites deemed to promote piracy. This could result in entire sites like Facebook and YouTube being blocked if any user-uploaded content violates copyright. SOPA is opposed by tech companies like Google and Wikipedia because it gives too much power to censor the internet and could negatively impact free expression. The document encourages readers to contact their representatives to voice opposition to SOPA and help protect internet freedom.