In that game, your goal is find out target number that chosen by the game itself. You are given 3 hints at the beginning of the game.

(+) sign denotes that; guess contains a digit which is also contanied by the target number and it's on the correct place. And (-) sign denotes that; guess contains a digit which is also contanied by the target number and it's on the wrong place.

Let's say, the target number ,which is chosen by randomly, 3467. If you try 1234 as guess, the game says that 1234 --. Both, target number and guess, contain 3 and 4 digits. However, they are on wrong place.
On the other hand, 9017 as guess gives user 9017 +. Because both, target number and guess, just contain 7 and its place is correct.