What is a DeFi index?

Can you explain in simple terms what a DeFi index is? I've heard about decentralized finance, but I'm not entirely clear on how an index fits into this ecosystem. Is it similar to traditional financial indices, or does it have unique features that cater specifically to the DeFi space? Additionally, how does one go about tracking or investing in a DeFi index, and what benefits does it offer to investors looking to diversify their portfolios?