Insert cards are the wild cards of any set — they're anything that isn't a base card or a parallel. Inserts come with their own unique themes, names, designs, and numbering schemes, and they're specifically "inserted" into packs alongside the regular cards.
Think of inserts as the bonus content of a product. They give sets personality and variety, and they're usually shorter-printed than base cards, which makes them more desirable. Whether it's a retro throwback design, a player highlight series, or a special achievement subset, inserts keep the ripping experience interesting.