] is a graphics directive which specifies that lines which follow are to be drawn with a thickness r. The thickness r is given as a fraction of the total width of the graph.
Thickness can be used in both two- and three-dimensional graphics.
The initial default is Thickness[0.004] for two-dimensional graphics, and Thickness[0.001] for three-dimensional graphics
See the Mathematica book: Section 2.9.3.
See also: AbsoluteThickness, PointSize, Dashing, PlotStyle.
Two ordinary circles and a thick one.
Evaluate the cell to see the graphic.