Aliases are simply just JSON objects where every key is an expression. They are used to generate new documents from existing documents.

  "username": "add($username, 'W')",
  "profile": {
    "description": "coalesce($description, 'No description provided')",
    "age": "add($age, 1)",
    "loves_cookies": "gt_eq($age, 1)",
    "best_friend": "$best_friend"
  "joined_at": "$joined_at"

As you can see, they can be nested objects. They are used all over the place in CookieDB, from selecting documents to editing tables.