The main window is the part of the BitTorrent interface that you'll likely be interacting with the most, so it's good to get familiar with it by reading this section of the help.
Several sections of the main window have column-based, tabular information. Note that left-clicking on a column will sort the items in the list by that column, alternating between ascending and descending sort for each additional left-click. You can select what columns you wish to see by right-clicking the column headers. If you wish to reset all changes you've made to the columns, you can select "Reset" in that context menu.