You take a new cast iron skillet, cover it with a fine layer of vegetable oil or lard, put it in your oven - upside down - and heat it up for one hour at 350 degrees and then let it cool down for several hours. The main purpose is to build a layer on the skillet to help prevent rusting but most cooks will swear that it helps the food taste better.   I've done the same thing with the grates for my smoker/grill and it works wonders.