To put it in a nutshell, the Traditional Catholicism forum is for discussing the traditional Latin Mass – also called the Tridentine Mass, the 1962 Missal, the Extraordinary Form of the Latin rite, etc. (Frankly, I wish the description of the forum mentioned that. I know, I know, there are lots of other issues involved besides the Mass-- but I just think it would avoid a lot of confusion for people if the forum description actually said “this is about the traditional Latin Mass.”)
To give you some rough idea, people who identify as “traditional” Catholics generally attend the Tridentine Latin Mass whenever possible, and encourage a return to various pre-Vatican II practices and devotions (such as prayers that are no longer common, a preference for older scripture translations such as the Douay-Reims, women wearing head coverings in church, receiving Holy Communion kneeling and on the tongue, etc.).