Silkscreens or artworks that are printed on the top and/or bottom of the PCB. The unknown electrical significance and are only for human consumption, mainly for use during manual assembly and for field service engineers. For cost sensitive PCBs the silkscreen will usually be omitted to say money.
Also a side-effect of silk screens when they are used to define part reference IDs is that the consume PCB area and can cause problems when you're trying to design a compact PCB.
