First class functions with callback

I am learning about functional programming, and have a question at this example in the book Mostly Adequate Guide:

// this line
ajaxCall(json => callback(json));

// is the same as this line
ajaxCall(callback);

From what I see, ajaxCall takes json as argument. But, callback is not a JSON. Part of the confusion is that I do not know what ajaxCall is – is it a generic placeholder here, or a established function in the wild?

How are the above two lines equivalent?