Formerly, TuD was using HTTP (HyperText Transfer Protocol). Now they are using HTTPS (HyperText Transfer Protocol Secure)
By using HTTPS, the communication between your browser and the web server is encrypted so that no one in between can read it. This helps keep your communication safer.
There may have been other changes that were made, but the switch to the secure protocol was the most obvious.
As you can see here, they are still not setting both the Secure and HttpOnly flag for all cookies, which would be preferable if the cookie does not need to be accessed by any legitimate JavaScript.
And as I mentioned here, they should also eliminate support for TLS v1.0.