The term "pre-seasoned" is used in reference to cast iron skillets and pots that have been treated by the manufacturer prior to sale. These products are typically marketed as being more consumer friendly, since they can be used immediately after purchase without the need for an initial seasoning. The process of pre-seasoning is similar to normal seasoning, in that it involves applying some type of substance to a cooking implement, and then heating it up. A number of different substances can be used in the pre-seasoning process, though vegetable oil mixtures are common. The quality of a pre-seasoned skillet or pot can depend largely on the specific process and substances that are used.
Cast iron is naturally gray in color, and does not take on the matte black finish commonly associated with iron skillets until it has been thoroughly seasoned and used. Seasoning has traditionally been performed by covering a skillet or pot in grease or lard and then heating it, which causes the oils in those substances to carbonize. Those carbonized oils cause the skillet to appear black, and also help to create a surface that is naturally non-stick. Cast iron implements typically get darker with increased use, and can actually get better with age if they are maintained properly.
Unlike traditional iron skillets that come from the factory untreated, pre-seasoned implements undergo a seasoning process prior to sale. The exact process can vary from one manufacturer to another, though one common method involves coating a skillet in a mixture of vegetable oil, and then heating it to a specific temperature. This has a similar effect to traditional seasoning procedures, since some of the vegetable oil carbonizes and turns the skillet black. Some pre-seasoning processes are inferior to others though, and result in a black surface coating that can chip off during use. High quality pre-seasoned skillets can perform similarly to untreated units that are seasoned after purchase, and may last just as long if they are maintained properly.
The main benefit offered by pre-seasoned cast iron skillets and pots is that they can be used to cook right away without any initial seasoning. Manufacturers sometimes use this as a marketing point to help these skillets compete with modern pans that have chemically treated non-stick surfaces. Some people still prefer to season their own cast iron though, and it is sometimes possible to remove the pre-seasoning from a skillet, and then season it the old fashioned way.