CORS is the way to share resources between different domains. while avoiding potential cross site scripting attacks. If you do not understand what CORS is or just want to learn more on it, I suggest you read one of the following or all if you're awesome:
- HTTP access control (CORS)
- Using CORS
- W3C Cross-Origin Resource Sharing
- Wikipedia- Cross-origin resource sharing
An understanding of this will definitely help you in future projets.