Hi,
The Offset is used to basically deduce or minus the height from the total height of the page. So for example your
Header is 40px height
Page is 840px height
If you set slider to fullscreen the slider will take 840px height of the page. Without the offset the bottom of the slider would be cut because the page is now 40px header height plus 840px page height . So its actually now 880px where as the page height is 840px.
So to make it line up correctly we set an offset so the slider deduces the 40px of header from page so the slider height is 800px and lines up perfectly on the page.
Hope this Explains Better.
Thanks
Tahir