what is the callback being passed?

var stream = require('stream');
var util = require('util');
util.inherits(Writer, stream.Writable);
function Writer(opt)
{
  stream.Writable.call(this, opt);
  this.data = new Array();

}
Writer.prototype._write = function(data, encoding, callback) {
  this.data.push(data.toString('utf8'));
  console.log("Adding: " + data);
  callback();
};

var w = new Writer();
for (var i=1; i<=5; i++){
  w.write("Item" + i, 'utf8');
}
w.end("ItemLast");
console.log(w.data);