A simple but fun mind exercise.

Your phone holds a 4-digit number in her mind and you are trying to guess it.

After each guess, she ("your phone") gives you some hint.

For example, if she holds 1234 and you make a prediction like 4567, then since only one digit is true "4" but it is not in correct place, it gives you a hint such as "-1".

If your guess is 2764, then since two digit values: 2 and 4 are right but only 4 is in its correct position, then the hint is like "-1 +1".

Thus, -n shows that you predicted n digits correct but they are in wrong places
and +n plus shows that you predict n digits correct and they are also in right position.

Have fun!