The objective is to correctly guess the 4 digits number generated by the computer.
A bull means that you have guessed a digit and it is in the correct position.
A cow means that you have guessed a digit but it is in the wrong position.
For example, if the is 5682 and your guess is 5870, then you have scored 1 COW and 1 BULL. The 1 BULL is for the digit "5" which, as you guessed, is in the first position. The 1 COW is for the digit "8" which is in a different position.
You can keep guessing till you find the correct number.