What Mr Red is describing is the difference between reactive and non-reactive metals for pans. When I pull out my instructions for various food preservation (canning) projects, invariably is the instruction "use a non-reactive pan." The grape juice I'm making into jelly or tomatoes I'm turning into sauce are going to react with the aluminum or copper.

From Martha Stewart.com, Reactive vs. Nonreactive Pans explains the difference.

A reactive pan is one that contains metals that might interact with certain foods. Aluminum, cast iron, and unlined copper are all examples of reactive metals. Pans made of these materials conduct heat very well and are ideal for cooking rice, melting sugar, and pan-browning meat. But you should avoid cooking acidic foods such as tomato sauce in pans like these, as the metal can alter the color and flavor of the dish.

Stainless steel and tin (including tin-lined copper) are examples of nonreactive metals. You can use these pans for all kinds of foods, though you may not get the heat conductivity of copper or cast iron.

There is really no single metal that is perfect for all foods, so it's best to buy different pieces to suit different needs (cast iron for skillets, copper for saute pans, etc.) rather than a cookware set if you can avoid it. If you do buy a cookware set, your best all-around option is stainless steel-lined aluminum.