Materials
Solder Mask
In plain English
The colored coating (usually green) covering the board that protects the copper and leaves bare only the spots where parts attach — like paint with a few gaps left on purpose.
The technical version
Solder mask is the polymer coating, usually green, applied over copper to insulate traces and prevent solder bridging during assembly. It defines which pads are exposed for soldering and must maintain minimum web widths between fine-pitch pads. Pads can be mask-defined or copper-defined depending on the assembly need.