It's your email, social account, ip/location, browser fingerprint info, search terms, information from their partners (ad networks, apps) and cookies, subs you visit, what you upvote/downvote/save/report, which page on reddit you're coming from/going to, etc. They use these to then determine blocks/shadowbans/counteract your votes and so on.
Has this resulted in a substantial quality increase on reddit? Oh absolutely not, you'll get chatgpt bots, people harassing you, completely unrelated comments, report abuse, etc. but they'll never give up that much data.