A good passphrase is a passphrase that:

1. is long enough that you cannot guess it easily with random trials (also called brute force attacks)
2. if made from elements, is random enough that you cannot guess it from random trials of elements combinations (also called dictionary attacks)
3. you can remember easily

There is a method that will help you make a strong passphrase with all those characteristics: the diceware method. Here is the website with an explanation, however do not roll dices online, this is not a good practice.

The method is the following: -pick up a long list of words, all indexed by a list of values. I personally use this one, made from texts from the Star Wars universe