How does D3 Object function pbi.dsv work? JavaScript colon functions

Im struggling to understand the following code because i dont know which function sends/gets which paramaters from where and why:

pbi.dsv ( 

  function(letters) {
    x.domain(letters.map(function(d) { return d.letter; }));
    y.domain([0, d3.max(letters, function(d) { return d.frequency; })]);
    svg.append("g")
       ...
  }

);

The object:

var pbi = {

   width:1200,
   ... ,

dsv:function(accessor, callback) {

    data = [ ... ];

    if (arguments.length > 2) {
        callback=accessor,accessor = null;
    } else {
        data = data.map(function(d) {return accessor(d)});
    }
    callback(data);
}

}

So which function is called first?

I know that pbi.dsv(...) calls the function from the object pbi.dsv:function (accessor, callback) { ... } and also retrieves the data from it.

But from where the parameters accessor and callback are filled? Does it get the data from PowerBI itself?

What exactly happens in this step. Which data fills the variable letters? Is it the result of pbi.dsv:function (accessor, callback) { ... }?

pbi.dsv(function(letters) { ... });

I couldnt find any documentation which explains this kind of functions to me and how they exactly work. I only understand that this would for example show an alert with the message "Hello". But still doesnt get how they work when parameters are filled inside the functions head. 

var obj = {
    test: function() {
        alert('Hello');
    }
};

obj.test();

I would thank You very much if you can explain me the syntax or a good website which explains it.