Non reactive pans and cookware are made of materials that do not chemically react with certain foods and additives.
Vinegars and acidic food used in pickling, canning and preserving usually require non reactive cookware. Even cooking of acidic foods such as tomato sauce or hot peppers in reactive pans is not really a good idea. All metals are reactive to one degree or another, copper, aluminum, and cast iron more so than others. Allthough these metals work very well as heat conducters and are ideal for many culinary endeavors, they are not good for pickling and canning.
Stainless steel is considered to be a nonreactive metal, allthough it does minimally react with foods the effect is so small as to be negligible. Glass and pyrex are the most non reactive material, but they do not conduct heat as well as metals.
Copper cookware that is lined with tin is sometimes touted as nonreactive. It is claimed the tin prevents chemical interaction between the food and copper pan. This quality is only short lived as tin is a soft metal, a few micro scratches and the food is exposed to the underlying copper. Enamelware is a step above coated copper so long as the enamel is not chipped or scratched.