PCB silkscreen is the writing, shapes, and lines that get printed onto a circuit board. Screen printing is used to specify useful information on the PCB board which can help users during the assembly. It is used to mark component values, part numbers, test points, polarity, etc. 

The graphics and text are usually printed with a permanent non-conductive epoxy ink. The color is typically white, although other colors are available.

Avoid generating Gerber files that have silkscreen over exposed copper pads. Most PCB manufacturers have software that will clip silkscreen that creeps over the line but it’s not always and not guaranteed. 

Silkscreen is either printed directly on the board or screen printed. Try to use lines that are wider than 6mils or else your graphics may get clipped by the fab engineer or be too blurry to read.