Reflection is a complicated topic that can't really be covered in a single recipe. However, a practical application of reflection is dealing with struct tags. At their core, struct tags are just key-value strings. You lookup the key, then deal with the value. As you can imagine, for something like JSON marshal and unmarshal, there's a lot of complexity for dealing with these values.
The reflect package is designed for interrogating and understanding interface objects. It has helper methods to look at kind of structs, values, struct tags, and more. If you need something beyond the basic interface conversion like at the beginning of this chapter, this is the package you should look at.