https://medium.com/rungo/everything-you-need-to-know-about-packages-in-go-b8bac62b74cc

If you are familiar to languages like Java or NodeJS, then you might be quite familiar with packages. A package is nothing but a directory with some code files, which exposes different variables (features) from a single point of reference. Let me explain, what that means.

Imagine you have more than a thousand functions which you need constantly while working on any project. Some of these functions have common behavior. For example, toUpperCase and toLowerCase function transforms case of a string, so you write them in a single file (probably case.go). There are other functions which does some other operations on string data type, so you write them in separate file as well.