This term relates to the removal of firescale from silver by use of chemicals, often slightly warmed.
When heated, silver blackens on the surface. It is common in silversmithing to pickle the object, bringing the shine back.
There are several methods.
Sulphuric acid is the most common chemical used. It normally is used in concentrations of 10% or less and often is slightly warmed in a soup warmer or other similar temperature controlled container
Hydrogen peroxide (sparex) solutions can be used, but are a bit slower. This also is used in a soup warmer.
Acetic acid, available from most chemists and home-brew suppliers, can be used but is so much slower that significantly long soaks are required.
It the silver is only mildly tarnished, it can be soaked overnight in household vinegar.