A legend is often printed on one or both sides of the PCB. It contains the component designators, switch settings, test points and other indications helpful in assembling, testing, servicing, and sometimes using the circuit board.
There are three methods to print the legend.
Silk screen printing epoxy ink was the established method. It was so common that legend is often misnamed silk or silkscreen.
Liquid photo imaging is a more accurate method than screen printing.