One of the significant and first ones is compound.

https://compound.finance/

Uniswap being another

https://uniswap.org/

A more comprehensive lost of ether based defi apps

https://www.blocksocial.com/ethereum-defi-platforms/

There are also other blockchain ecosystems with their own defi projects.

In general, you have some cryto assets, you loan those to someone or some decentralized system that uses those assets to make money(like banks, financial institutions, and market makers).