.populate() in Mongoose allows you to populate a reference you have in your current collection or document with the information from that collection. The previous may sound confusing but I think an example will help clear up any confusion.

The following code creates two collections, User and Post:

var mongoose = require('mongoose'),
  Schema = mongoose.Schema

var userSchema = Schema({
  name: String,
  age: Number,
  posts: [{ type: Schema.Types.ObjectId, ref: 'Post' }]
});

var PostSchema = Schema({
  user: { type: Schema.Types.ObjectId, ref: 'User' },
  title: String,
  content: String
});

var User = mongoose.model('User', userSchema);
var Post = mongoose.model('Post', postSchema);

If we wanted to populate all of the posts for each user when we .find({}) all of the Users, we could do the following:

User
  .find({})
  .populate('posts')
  .exec(function(err, users) {
    if(err) console.log(err);
    //this will log all of the users with each of their posts 
    else console.log(users);
  })