Decentralised Finance (DeFi) is a new financial system built on secure distributed ledgers. The system removes the control that banks and institutions have over money, financial products, and financial services.

With DeFi, people can do many things that banks do, such as borrow and lend money or assets, earn interest, buy insurance, and trade assets. But, these happen in a way that is faster and does not involve paperwork or a third party.

DeFi, like crypto in general, is global, peer-to-peer, and pseudonymous. It is not routed through a centralized system like banks are. Instead, it allows several entities to keep a copy of the transaction history that happened on it.
This is important because centralized systems (banks) and gatekeepers (bankers) can restrict transaction speed and sophistication. They also limit the control that users have over their money.

DeFi is unique because it extends the use of the blockchain beyond the simple transfer of value to more complex financial use cases. It allows developers to build more open, free, and fair financial markets available to anybody with an internet connection.
https://alabiansolutions.com/blog/a-simple-guide-to-understanding-defi/