Bulls - Bulls-eye, position matches
Cows - Position does not match
Cows are partial matches. If a guess has 2 cows, that means two of the letters in the guess are present in the hidden word, but at a different position.
The first guess ROSE tells that two letters match, one is an exact match Bull and other is a partial match Cow.
The second guess SEND has no matches, so letters S and E are eliminated, leaving O and R.
The next two guesses tell us that the hidden word starts with letters WOR. The final guess is WORK which is the hidden word.