Seasoning refers to a baked-on oil coating. When baked onto the pan, the oil turns dark and carbonizes, creating the non-stick coating cast iron is known for.
Only pans with exposed cast iron need seasoning, or pans only partially covered in enamel. The exposed cast iron will wear down over time, especially if it's been over-cleaned.
To check if your pan needs to be seasoned, check for rust and look at the color. A seasoned pan will be deep black and shiny, while an unseasoned one will look like gray metal.
You also need oil with a high smoke point that can be carbonized. Canola is ideal, with a smoke point of at least 400 degrees F, but grapeseed or sunflower oil will also work.
If your pan has rust, take a little steel wool and scrub until you’re left with clean metal. You can also scrub the pan using coarse ground salt and a rough sponge.
Clean your cast iron with soap and water. Be sure to get all the debris off and thoroughly rinse to ensure no soap residue before you carefully dry your pan.
Coat any exposed cast iron in a light, thin layer of oil. The pan can be sprayed with the oil, brushed on using a pastry brush, or wiped on using a cloth or paper towel.