A passphrase, rather than being “just” a password with more or less random characters, is a sequence of words strung together that can make for a very good access credential while at the same time being easy to memorize. So, in a way a passphrase “is” just a password, albeit a very special kind. When one really thinks about it tough, the best way to think of a passphrase is a way of constructing a clever password.
The key idea is that a passphrase can be long, complex, and have a high entropy, while still being easy to remember for a human as it contains a certain meaning. To really work as a password tough, such passphrase may have to be enriched with numbers and special characters. At the same time, any predictable patters or words that are too common should be avoided.
A good passphrase could, for example, combine a few things that just happen to be in the office of the user plus a few additions. In the author’s case, such phrase might be constructed from the words “book, window blinds, water, and sunglasses”. With a few common substitutions the result might be “b00k!windowblinds!water!Sunglasses”.
If you want to learn more on the topic, you might be interested in Engity’s article covering passphrases, how to construct and how to use them.