I am soooo confused on interest rates and all…!

I just read this old post: https://www.reddit.com/r/ethereum/comments/c4p19w/no_loss_lottery_on_ethereum_launching_today/

​

And learned that you can put your eth in a "pool" and earn interest. What? How does this work?

Can you actually "lend" eth to someone/thing and get interest on it? How can you make sure you will get your money back + interest?