hello-world.ts(4,1): error TS1208: 'hello-world.ts' cannot be compiled under '-- isolatedModules' because it is considered a global script file. Add an import, export, or an empty 'export {}' statement to make it a module.

I hadn't added the export keyword in front of the function. That was the reason of this error.

Here I want to understand the "meaning" of this error message.
It mentions "global script file".
What does that mean?