Seasoning is a baked on coating of fat that protects the cast iron from rusting and provides a reliable cooking surface.
Once again, very important that you don't use soap or scourers once it has been seasoned. If hot water and a stiff brush won't get it clean, use a little kosher salt and that brush. Cast iron, and carbon steel (woks, and Lyon pans, some crepe pans) use the same techniques.
I do like to give mine a little wipe with oil ewvery now and then as I don't use mine daily. (I am a pan junkie and couldn't use all pans each day if I tried!)