The Great Firewall is CERia's regulation to try and censor the internet in their country. While on the surface this is meant to block erotic and excessively violent information, in reality it's used by the government to censor any opposition to them and hide their corruption.
Despite this, the Great Firewall isn't impenetrable and savvy hackers, like Frey and Vance, can easily get around it to access "forbidden" information.