Cast iron cookware has a porous surface and must either be oiled before cooking or seasoned.
Seasoning is a process by which a layer of animal fat or oil is applied and cooked onto the cast iron to fill and smooth the surface to make it smooth and non-stick. A seasoned cast-iron skillet has a stick-resistant coating created by polymerized oils and fats. Aside from the benefits mentioned before, the seasoning layer protects the cast iron cookware from rusting and prevents food from interacting with the iron of the pan.
Traditionalist cast iron users would use lard or shortening. Some used animal fat, butter, flaxseed oil, grape seed oil, and even bacon grease as cast iron seasoning. They say that flaxseed oil is ideal for cast iron seasoning because it is drying oil, which means it can transform into a hard and tough film that will give your cast iron that smooth and non-stick surface.
Regardless of what type of oil/grease you are going to use, take into account that you must coat every nook and cranny of your pan evenly to have a nice layer of seasoning. It is also recommended to heat your cast iron cookware slightly before applying your choice of oil to ensure that the cast iron skillet or pan is completely dry and the pores are open before seasoning.
A note to ponder, the non- stick properties of the cast iron cookware will increase with time and use, so you have to use your cast iron often. Seasoning your cast iron is a process and your cookware will get better over time.