The weekly trigger allows you to trigger a build at a certain time, on a certain day and at certain intervals.
The time can be entered in manually or specified via the time picker. Setting values in the time picker is as easy as typing in the value, alternatively you could use short cuts which are explained on the main Time Triggers page. After selecting a time, you can then select a day the trigger runs on and specify the weekly interval. A value of 0 or 1 means it runs every week, a value of 2 means it runs every fortnight, etc...