From the article: "The abstract operation FunctionDeclarationInstantiation takes arguments func (a function object) and argumentsList. func is the function object for which the execution context is being established."

They further explain the steps going on.

In my own words this is the process of "setting up the environment which the function later runs in". This includes some setup work with the values passed as arguments concerning their default value, if one is present, or the value given in the call to the function.