What are Trustlines?

When you hold assets in your Stellar wallet, you're actually holding credit from a particular issuer. The issuer has agreed to trade you a digital representation credit on the Stellar network for the corresponding asset in the real world.

To receive or hold an asset issued by an anchor in your Stellar wallet, you must establish (or create) a trustline to that anchor's asset first. Doing this signifies that you trust the issuer to allow you to redeem the real-world asset at a later date by trading back the digital representation. Trustlines also protect you from having random and unwanted assets sent to you.

For more info: https://developers.stellar.org/docs/issuing-assets/anatomy-of-an-asset/#trustlines