Provide collateral and take out a loan against that collateral. It allows people to act as their own bank. No longer do you have to go to a bank, ask for permission and then get approved for a loan. Now, you can do that yourself, instantly, without any trouble at all. Amazing really.
What are those loans used for today? Well, mostly it is about interest rate arbitrage and providing liquidity. As a super basic example, you can borrow funds at 2% and then lend them out again at 3% and make 1%. It is essentially risk free (assuming the contract doesn't have bugs/exploits).
The larger picture will be to enable people to be their own Kiva's. Crypto often is pushed to 'bank the unbanked', but it is more than just holding money. It is enabling people to borrow against their existing holdings, effectively allowing anyone, globally, to put their savings to work for them, without having to rely on a centralized banking system to do so. This might not be interesting for USA people, but it is especially valuable in countries that don't have a stable banking system.