Could someone explain this function to me?

var createManyPeople = function(arrayOfPeople, done) {
    Person.create(arrayOfPeople, function(err, data){
      if(err){
        done(err);
      } else {
        done(null, data);
      }
    });
   };

what is the “done” function here?

Done looks like a callback function for when the Person.create function is done with errors or without errors

Sure, so take a look in the very first line:

var createManyPeople = function(arrayOfPeople, done) {

In that line, we have two parameters that are going into the function: an array of people, and a callback function to run at completion. done is a function we can pass in (remember, in javascript, functions are treated like data, just as strings or numbers or arrays, so we can chuck 'em around anytime!).

This is a pretty common trick, to allow a function to be passed in and then to call that function when a given condition has been met.