What is a virtual for an URL in Mongoose?

I am currently following a tutorial on implementing a database (Mongoose) in a simple website created using Express-framework. I do not have any problem understanding the concept of models, but I fail to make sense of the lines following the comment "Virtual for book's URL" in the code attached. How do these lines operate, and what role does having a virtual property have in this context?

var mongoose = require('mongoose');

var Schema = mongoose.Schema;

var BookSchema = new Schema(
  {
    title: {type: String, required: true},
    author: {type: Schema.Types.ObjectId, ref: 'Author', required: true},
    summary: {type: String, required: true},
    isbn: {type: String, required: true},
    genre: [{type: Schema.Types.ObjectId, ref: 'Genre'}]
  }
);

// Virtual for book's URL
BookSchema
.virtual('url')
.get(function () {
  return '/catalog/book/' + this._id;
});

//Export model
module.exports = mongoose.model('Book', BookSchema);

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