The game is played by guessing a number and the opponent mentions the number of Cows (for the digits which are present in the secret number, but not in the guessed positions) and Bulls (for the digits which are present in the secret number, and also in the exact positions).
For example, if the secret number is 1234, and the first guessed number is 5678 would result in "0 Cows, 0 Bulls" (since all the guessed digits are not in the secret number). If the guessed number is 4567 it would result in "1 Cows, 0 Bulls" (since 4 is present, but not in the exact position), and if the guessed number is 8214 it would result in "1 Cows, 2 Bulls" (since 2 and 4 are in the right positions, while 1 is in the wrong position). The game would continue until you guess the right number with "4 Bulls".