De means Decentralized.

We are escaping from the traditional way of centralizing all activities. 

Activities include the following 

• Trading
• Currency
• Lending
• Borrowing
• Remittances
• Etc

All these are aimed at being Decentralized.

Fi is short for finance. 

Finance entails all activities involving money. 

DeFi is Decentralized Finance

Advantages of Decentralisation of Finance Include. 

• Anonimity.  You may do business with a person you do not really know much about them. Pseudonyms are accepted here. 
• Large sums of money are transferred without any serious paperwork and passing through many hands. So it is permissionless. 
• You cannot censor DeFi transactions.  They can be traced but are censorship resistant. 
• You do not need to trust anybody so as to conduct trade with them. Decentralisation through blockchain keeps own data. 

Disadvantages

• Chances of hacking are very high. 
• Irreversible errors are prone in DeFi
• More loses experienced.

DeFi despite the few challenges has easened the way transactions are done.