The main window is the place where the alarms are managed.
The main window has two different views. The first is the default, with two calendars on the right. The other (old) view shows two lists of coming and past alarms. This view will be removed in a future version. The view can be set from the 'windows->view' menu.
Both views allow you to set the sizes of the different parts by dragging around the dividers. If you somehow get into a situation where you cannot undo the changes you made, you can reset the sizes to the defaults by clicking 'window->view->default window settings'.
The calendar view
In the upper left part of the window you see a list where the alarm groups are shown. The alarm groups contain the alarms and can be saved to a file. The list displays the names and certain properties of all loaded alarm groups. After selecting one or more alarm groups, you can perform an action on them from the alarm groups menu.
If you select an alarm group, the alarms in it will be shown in the list on the lower left. The list shows alarms in the selected alarm group. After selecting one or more alarms, you can perform an action on them from the alarms menu.
You can change the information which is shown in the lists in the 'Main window' tab in the settings.
On the right side of the window there are two calendars. More information about the calendars can be found here.
The old view
The old view shows the alarms list in the middle of the screen, with two additional lists of past and coming alarms on the right. There is also a box where you can enter notes.