The "checksum" that both of the two replies mentioned is the keyword.

Without going technical and disregarding the Base58 decode part:
It's basically the last few characters of the WIF private key, that was created by hashing the other parts of the prv key and it will be inserted at the end as a the checksum.
So changing a character from the first few characters of the private key will produce a different hash that will mismatch with the checksum.
Or changing a character from the last few characters (checksum) will make the checksum different than the hash of the rest of the characters.
A mismatch means an invalid WIF (Wallet Import Format) private key.