jkulubya's comment is largely correct. And you most likely do understand what an index fund is. It's just that most people don't realize there are two sides to the product - the theoretical and the real. The index is a theoretical product (intellectual property). The fund is the real-world implementation. A fund manager takes a look at one of our S&P products and says "I want to make a fund off this" and S&P contracts a license with them to allow it since S&P owns the IP on that index - it is S&P's design and methodology.

An easy way to think of this is the retail example where you pay an investment advisor. You pay them to manage your money but they place all the trades through some broker. S&P is the investment advisor and the fund manager is the broker.