Trustless means that you don't have to trust anyone.
When you deposit into a bank account, you trust that the bank will give back your money when you want it. You are also trusting that the government won't freeze your funds.
When you put money into many of the popular "defi" protocols, you are trusting that those in control of the protocol won't steal all of your money.
When you deal with a truly trustless defi protocol, you don't need to trust anyone. The smart contract code has the final say in what happens to your money, and that code gives that control to you. Not to an admin key, not to a multisig, not to a price provider, and not to a government.
A great example of a completely trustless defi protocol is WETH. The WETH contract allows anyone to exchange Eth 1:1 for WETH at any time, and at any time to exchange WETH for Eth 1:1. There are no admin keys, no multisigs, no unlimited governance upgrades, nothing except a decentralized blockchain.
This is how Trustless protocols are built.
What does "Trustless" mean? (this page)
And what is lending in defi anyways?
Trustless docs homepage