A registration key (or activation key) is used to unlock the full version of the game after the trial period ends. Butterfly Escape - Big Fish Games 13 Jul 2014 —
The safest way to obtain a key is through authorized digital retailers. Avoid "keygen" or "crack" websites, as these often bundle malware with their downloads.
The search for a "Butterfly Escape registration key" implies an intent to unlock the full version of a puzzle game. While the game is older, obtaining a key through unauthorized channels (warez sites, keygens) is not recommended due to high risks of malware infection and legal implications.
The Archive.org software library often contains disk images or "cracked" versions of 2000-era casual games for historical preservation.