How to fix "CORS policy: No 'Access-Control-Allow-Origin'" error?

I’m trying to fetch data from another domain using JavaScript, but I keep getting a CORS policy error. What does this mean and how can I fix it?

That error means your browser blocked the request because the server didn’t allow it. The server needs to set the right CORS headers to permit access from your site.

If you control the server, add an Access-Control-Allow-Origin header. If not, you may need to set up a proxy or contact the API provider for access permissions.