DATEDIFF(wk,0,GETDATE())
this will give you the difference between current date and 1/1/1900 in weeks(wk stands for week difference)
so for today's date(oct 5, 2012) the difference is 5883 weeks .
Now when you say
DATEADD(wk,DATEDIFF(wk,0,GETDATE()),0)
this will add the weeks to date 1/1/1900 and the out put will be start date of the week 5883 i.e. '2012-10-01'
and finally when you do a convert of 103, it will display output as '01/10/2012'