Offset[dx, dy, position] gives the position of a graphical object obtained by starting at the specified position and then moving by absolute offset dx, dy.
Offset can be used to specify offsets in any two-dimensional graphics primitive.
position can be either x, y or Scaled[x, y, ... ].
The offset is measured in units of printer's points, approximately equal to of an inch.
Offset[dx, dy] can be used to specify an absolute radius in a Circle or Disk object.
See The Mathematica Book: Section 2.9.4.
See also: Scaled, AbsolutePointSize, AbsoluteThickness.