Understanding the Language of Creatures is one of the spells Gwydion can learn in King's Quest III. It allows him to hear what animals are saying in the game. The spell is not required to complete King's Quest III.
After gaining access to Manannan's spellbook, The Sorcery of Old, Gwydion learned of this spell. By casting it, he was able to understand the speech of various creatures on his journeys from the cat in Manannan's house to two mice on the pirate ship that discussed the pirates' plans to bring in a cat to deal with them and revealed the location of the pirate treasure.
Creatures see and hear much in this world, and they understand more than we give them credit for. Knowledge may be gained by one who keeps ears and mind open. Of course one also hears a lot about worms and nuts and everyday life of mice. As one may have guessed, spells have both good and annoying aspects; this one won't let you talk back so you can't tell those chattering chickens to shut up.