It's the abbreviation for Decentralised Finance. Many projects are taking market share from centralised exchanges via decentralised finance such as decentralised exchanges, automated market making and liquidity providing, non custodial stable coins (usdt) savings accounts. All these projects bootstrap with their own tokens rewarding their users and keeping little to themselves contrary to centralised exchanges.
Google "DeFi guide decentralised finance" and you should find a bunch of videos and guides.
Be careful investing here since there are many scams and overvalued projects.