Non-reactive pots, pans, and other cookware equipment does not react to acids in food being cooked or marinatd in the equipment. Stainless steel, glass (such as Pyrex) and ceramic glazed cast iron (such Creuset brand cookware) are non-reactive. Cast iron and aluminum that are not coated with another material are reactive. It is important to use non-reactive cookware when using acidic foods such as lemon because reactive cookware may impart a taste to the food when it comes into contact with acid.
Here is a great explanation of how to tell the difference between aluminum (reactive) and stainless steel (non-reactive) cookware.