There are 4 options to filter threads - latest, top, newest, oldest
latest is the option if you are interested in seeing threads that have had recent updates
Newest sorts by when the topic was created. Newest threads are at the top.
Oldest is the opposite of newest. Oldest thread is at the top .
I think top sorts threads by number of responses. The thread with the most responses is at the top.
I generally am interested in seeing new updates to threads so I use the latest option