Here I defined a function via a function expression and I assigned that function to a variable called func. I can then call that function and the string hello world is returned. This is the basic idea of what is accomplished with many functions, you call it and some kind of product is returned. This might be a silly pointless example, but we will be progressing on to some real examples later on in this post that also return a product when called.
In other words something like this.
Pure functions are functions that only work with what is given via a set of arguments, and not anything that that is a global variable of object class instance. They also do not mutate and object that is given in place but return a new object.