The goal of Moo is to guess a random number the game has generated. This number is defaulted to four digits, but you can adjust this in the Settings tab.
Each time you make a guess, the game will give you a result. If you guess it correctly, it will say "You win!". If you guess it incorrectly, it will give you a result using the letters "A" and "B". This result is a hint to guide you to the correct answer.
An "A" means you have a right number in the right spot. A "B" means you have a right number in the wrong spot.
You can also read the rules on Wikipedia. The game is called "Bulls and cows", and one of the first computer implementations of it was called "moo", hence the name of this game.
No cows were harmed in the production of this app.