Seasoning is the layer of polymerized oil bonded to the surface of a cast iron pan. When you heat thin oil past its smoke point, it goes through a chemical change called polymerization and turns into a hard, plastic-like coating that bonds to the metal. That coating is what makes a well-used cast iron pan release food cleanly and resist rust.
The part nobody tells you up front: seasoning is not one-and-done. It is a layer you build and maintain over the life of the pan. Cooking fatty foods adds to it. Scrubbing it with harsh detergent and leaving it wet strips it down. A pan that looks dull grey and patchy is not ruined, it just needs re-seasoning, which is one of the reasons cast iron is so forgiving compared to coated nonstick that you simply throw away when it wears out.
If a pan ever feels sticky or tacky rather than smooth, that is usually too much oil applied too thickly during seasoning, not a defect. Thin coats, fully heated, are the whole secret.
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