The easiest way to access censored websites in China is to use a VPN. Short for virtual private network, VPNs are subscription services that encrypt your internet traffic and route it through an intermediary server outside of China. By installing a VPN app on your computer and connecting to a server, you can bypass the Great Firewall and freely access the web.
Websites are censored in China at the behest of the government and the ruling Communist Party. Any websites or apps that undermine Party rule, or have the potential to, are typically blocked. This consists largely of western news media, social networks, and sites built on user-generated content. Other content deemed vulgar, pornographic, paranormal, obscene, or violent is also blocked. Some western websites, apps, and services are blocked in order to prevent competition with domestic, homegrown alternatives.
Comparitech maintains an updated list of VPNs that work in China here. Sometimes VPN servers get blocked, especially during times of social unrest and international conflict. That’s part of the reality of living in China, but for the most part the VPNs in that list are the best options.
The "Great Firewall" is a colloquial term for mainland China's internet censorship system. It's part of the Golden Shield Project, also called the National Public Security Work Informational Project. Both legislative actions and enforcement technologies are used to regulate the country's internet. The Great Firewall blocks foreign websites, apps, social media, VPNs, emails, instant messages and other online resources deemed inappropriate or offensive by authorities. This ranges from vulgar content such as depictions of violence and pornography to more politically-sensitive materials that promote democracy or depict the ruling Communist Party in a poor light. Western social media (Facebook, Twitter), user-generated content sites (Youtube), and tools (Gmail) are blocked almost wholesale unless they agree to comply with Chinese laws and regulations. A combination of technologies are used in combination by government-run internet service providers and domestic internet companies to censor content, including keyword filtering, IP address blacklists, DNS poisoning, packet inspection, and manual enforcement.