Private Key – What exactly is the one word seed word?

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 to your Bitcoin private key. Surprisingly, I see some transactions using that private key.

The string in the word variable contains one BTC private key.

My question is – what is the relationship between a string and a Bitcoin private key? Could you please make it simple for me?

Also, why do people talk about 12 word seeds? Here one word is converted into a private key.