Solder mask, also known as a solder resist or solder stop is a thin layer of polymer that is applied onto the copper traces of a PCB to prevent oxidation when exposed to the environment and to protect the copper from contaminants like dust and dirt.
The mask also aids in achieving proper soldering results by minimizing the chances of solder bridges forming between closely-spaced solder pads.
A solder bridge is a connection between two conducting parts of a board through a small, unintended blob or whisk of solder.
You won’t always find solder masks on hand-assembled circuits, but they’re an essential component for mass-produced boards that are soldered automatically using solder-bath or reflow techniques.
In electronic design, the solder mask is treated as a separate layer of the PCB, similar to the copper and silkscreen layers.