Here's the simple summary:
Friction, inaccessibility, and regulatory uncertainties are some of the major issues that arise with CeFi (centralized finance). Not everyone is privileged enough to be banked and it's hard for them to compete on an even playing field.
DeFi is meant to provide financial tools without any restriction on race, religion, nationality, or geography. It's the closest thing we currently have to make the "right to be banked" a birth right.
Defi, or Decentralized Finance, is the movement that allows individuals to utilize financial services such as borrowing, lending, and trading without the need (or permission) for a centralized entity (no Bank of America, Chase, etc).
The financial services are provided via Decentralized Applications (Dapps)...a lot of them are deployed on the Ethereum blockchain but there are an increasing number of other blockchains with Dapps. All come with their own pro's and con's.
DeFi is not a single Dapp...there is a whole ecosystem of Dapps that allow users access to banking, insurance, bonds, money markets, etc.
Because of their decentralized nature, people can enter and leave as many or as few Dapps as they please...they're often called money Legos.