The S stands for “secure”, specifically it means that the connection between your browser and the website is encrypted using the TLS protocol.
The TLS protocol does two things:
1. When you connect to the server, the server sends you a digitally signed certificate. The certificate is used by your browser to validate the server’s identity, i.e. make sure that the entity sending you the certificate is indeed reddit and not a hacker pretending to be reddit.
2. After validating the certificate, the client (your browser) and server set up an encrypted connection, so that all the data transmitted between them is encrypted. This means that it is hidden from anyone who intercepts the communication, and that it can’t be manipulated.