Mojibake (Japanese: 文字化け), literally 'character transformation' is a phenomenon where text that is legible in one character encoding is read in another character encoding and is made illegible.
In the Pokémon games, each text character has an index number, but the index numbers for characters and character sets may differ between versions.
An example of mojibake is the text "(x)Df-", which is used for Pokémon Gold and Silver Generation II error codes in the English versions.
The 'Df-' string is encoded from the indices 83h, A5h and E3h and is illegible in the English versions. In the Japanese versions, they make sense when the indices are put together, because this brings up the text 'エラー' (meaning error).