The keys to spending your bitcoin are in the form of a string of numbers and letters that are somewhat random in nature. A human would have a very hard time remembering it. Instead, a specific list of 2048 English words are used to create this string of numbers and letters through a special cryptographic 1-way method called a hash. The most common arrangements of these words are in 12 or 24 word combinations.
These 2048 words are very unique. to the fact that only the first 4 characters of each word will determine the entire word, regardless of how many characters are in the word. It is also engineered that the last word (either the 12th or 24th word) is a special word that ‘verifies’ the prior words are correct by a special check so that bitcoin software can verify that it is a valid bitcoin seed phrase.
In the same way the keys to your house would allow anyone who has a copy of them to enter your house, your seed phrase will allow anyone with a copy of it to spend your bitcoin.
A 12 word seed phrase can easily be memorized. Why would you want to memorize it? Generally you shouldn’t because memories fail, an accident can happen and cause amnisia, a stroke that causes permanent memory loss, etc. So now with those negative thoughts out of the way, why would you want to memorize it? If you are in a hostile environment and you are running away to escape the hostile environment, you can memorize it and leave. Once you get to a safe location, you can recover your bitcoin and avoid starting from nothing.