The default sort option for topics on the board is by date of the last post in that topic, i.e. the Last Post column, which if you look at the header has a small white triangle alongside the title. The triangle shows it's the default sort option.
Now the neat thing is that forums can be sorted by *any* criteria, so if you want to see all topics sorted by username you click the Started By column header and the topics are rearranged for you and the white triangle is now at the top of that column.
Where it becomes even more useful to find those unreplied to topics is to click on the Replies column and the topics are sorted with the 0 reply posts at the top, working your way down to those with 1 reply, then 2 replies and so on....
You may have spotted that the triangle is sometimes point upwards and other times is point downwards... this indicates the sort order... so clicking the Replies column a second time will give you the topics with the most replies at the top. Similarly clicking the Posted By column a second time gives the posters names in reverse alphabetical order.
All five of the column headings work in this way, so you can view the board how you want to. Note: this is a variant of the default behaviour so will revert back to the default view once you post a reply or leave each forum.