This is a simple implementation of mnemonic seed phrase generation and recovery as described in BIP39.
This code is meant to be education and is not hardened for production use. It has not been audited or thoroughly tested, and most error handling has been omitted.
For simplicity's sake, this code directly uses the first 32 bytes of the seed as a private key. (It doesn't do BIP32 or the like.) This is another reason not to actually use this code aside from its intended purpose: explaining how BIP39 works.