DeFi is decentralized finances, a way of lending and taking loans on Ethereum network in decentralized way.
Right now we got into 3 million ETh locked.
Why people do that? As they make passive income without trading in a pretty secure way. There is whole ebook about passive income from cryptocurrency to download for free too.
While in whole DeFi we have even more and thats what the book touches also
If you want to also check non DeFi but good ones ie run by Gemini Exchange look at best crypto lending and loan offers list where you have plenty ways to make passive cash or get a loan in crypto.
Ethereum Top10 Markets
It is what it is though.