If you're brand new to DeFi and need a thorough overview of the big ideas and major concepts behind Decentralized Finance you should take some time to read through Ethereum's DeFi page.
While it's written with the Ethereum ecosystem in mind, it teaches you the core principles and topics common to the Open- and Decentralized Finance movement, no matter which Blockchain it's built upon. Also make sure to follow the links which will guide you towards more in-depth articles and blog posts on topics such as Quadratic Funding or Flash Loans.
- Ethereum Foundation