Warning: in nearly all cases, the use of a brainwallet is extremely high-risk/stupid. Do not use this tool unless you have a clear need and understand the risks.
A brainwallet is a Monero address and keyset derived from a user-defined passphrase. This allows the seed to literally never be written down or typed, and therefore ensures (in theory) that the funds in the associated address are safe forever within the brain of the creator. In reality, humans are shockingly poor sources of entropy and the phrases that they create are notoriously easy to crack (albiet far more difficult in Monero than Bitcoin due to the underlying architecture). Nevertheless, in very specific scenarios and use cases they do have a role to play. If you are unsure at all or are unaware of the risks - DO NOT USE A BRAINWALLET.
Here is a good talk on how someone managed to gain access to large numbers of Bitcoins by exploiting poor entropy in brainwallet creation in Bitcoin. Don't be a statistic :)
This tool can be found online at monerobrainwallet.com.
This tool was made by taushet on as a fork of Luigi1111's toolkit. If you want to donate Monero to support the core development team, send it here: donate.getmonero.org.