private key – What is one word seed word exactly?

I have the following code

import hashlib
import binascii

def word_to_private_key(word):
    sha256_hash = hashlib.sha256(word.encode()).digest()
    private_key = binascii.hexlify(sha256_hash).decode()
    return private_key

word = "hello"
private_key = word_to_private_key(word)
print("Bitcoin Private Key:", private_key)

Here hello is converted into bitcoin private key. Surprisingly I see some transactions with that private key.

For any string of the word variable there is one btc private key

My question is – what is the relation between string and bitcoin private key?
Could you make me understand in simple way?

Also why people talk about 12 word seed? Here one word is converted into private key.