An offset is the difference detween a point and another reference point.

If we say that the top left hand corner of the screen is offset by 100,100 from the top left hand corner of a map it means that the top left corner of the screen will display the information at position 100,100 of the map.

Of in otherwords
Code:
 123456789
1AAAAAAAAA
2AAABAAAAA
3AAAAAAAAA
The B in the disagram is offset by X=4; Y=3 within the image.

Does that help?