Scaled[x, y, ... ] gives the position of a graphical object in terms of coordinates scaled to run from 0 to 1 across the whole plot in each direction.
Scaled[dx, dy, ... , , , ... ] gives a position obtained by starting at ordinary coordinates , , ... , then moving by a scaled offset dx, dy, ... .
Scaled can be used to specify scaled coordinates in any two- or three-dimensional graphics primitive.
You can use Scaled to represent objects that occupy a fixed region in a plot, independent of the specific range of coordinates in the plot.
See The Mathematica Book: Section 2.9.4 and Section 2.9.10.
See also: PlotRange, PlotRegion, Offset.