In the past months, these services seem to be on the rise. Instead of keeping your crypto in your wallet, you store it at these services. They use the funds to give out loans and reward you with daily or weekly interest payouts. The rates vary from cryptocurrency to cryptocurrency, with USDT and USDC generally giving the highest rates. The value of these are pegged to the USD and don’t see crazy fluctuations like Bitcoin, for example. However, should the USD collapse, so will your USDT or USDC.
See also here, diversification is key. Don’t buy only Bitcoin, there are many big coins out there that are worth a look. I’d advise everyone to sign up to Coinbase and click the ‘Learn’ link to learn more about the big currencies and how they work before you decide which to buy.