GrayLevel[level] is a graphics directive which specifies the gray-level intensity with which graphical objects that follow should be displayed.
The gray level must be a number between 0 and 1.
0 represents black; 1 represents white.
On display devices with no native gray-level capability, dither patterns are typically used, as generated by the PostScript interpreter.
The end of the Graphics Gallery shows examples of gray levels generated with various GrayLevel specifications.
See The Mathematica Book: Section 2.9.1 and Section 2.9.3.
See also: RGBColor, Hue, Raster.