What does this term mean?

Subscribed threads are threads in which you can go to MyRCU at the top of each page, and view just the threads you're interested in following (subscribed to). It's a way for you to keep track of certain threads without having to search for them. Just go into MyRCU, and those threads will be right there for you to read everytime.

Some people have their preferences set in MyRCU to subscribe them to all threads they have participated in, while others subscribe to threads based on their interest in the topic.

Hope that helps a little...