This is one of many tinctures. Tinctures are the colors, metals, and furs used in heraldry, though the depiction of charges in their natural colors or “proper” are also regarded as tinctures.
In heraldry, argent is the tincture of silver, and belongs to the class of light tinctures, called “metals”. It is very frequently depicted as white and usually considered interchangeable with it. In engravings and line drawings, regions to be tinctured argent are either left blank. Sometimes it may be indicated with the abbreviation “ar”.