What does DeFi mean?

Sure, here's a description in the tone of a questioner: "Hey there, I've been hearing a lot about DeFi lately and I'm curious to know what it means. Could you give me a quick rundown? I've heard it's some kind of financial system that's decentralized and built on blockchain technology, but I'm not entirely sure how it works or what its benefits are. Can you help me understand the basics of DeFi and why it's becoming so popular in the world of cryptocurrency and finance?