Not sure if trolling but, it is basically what the word means.
A wallet stored in your brain in the form of 12 words passprhase (as usual).
You basically create a private key to a wallet using a 12 word passprhase.

Remember, 12 words is what is usually used, you can use less or more.
I would advise you to use more and like others said, use words not published anywhere before.
Use slang if you wish, just make sure it's a word not published anywhere before.


EDIT: here: https://brainwallet.github.io/