A passphrase is any word and/or number combination you make up yourself. As long as your passphrase is stored only in your head it will be impossible for anyone to know what it is.
This is also known as a 25th word (or 13th word if using 12 word seed).
It looks fine when you're still healthy, but if you suffered a brain injury regardless what the causes e.g. getting old, accident, crash etc, using passphrase isn't safe anymore. Imagine you're the only one person who know the last word, there's no back up and you can't do anything except let it go.
Brain Injury Awareness Month is a major event in the public health space. There are more than 5.3 million individuals in the United States who are living with a permanent brain injury-related disability. That’s one in every 60 people. At least 2.8 million Americans sustain traumatic brain injuries in the United States every year. As a result, Brain Injury Awareness Month rallies a large community into raising awareness and advocating for causes they care about.