In that model of integers a balloon represents +1 and a weight -1 (because a balloon in real life goes up into the sky, and a weight goes down to the floor).
Then, blank 1 is +1 and blank 2 is -1.
Adding a balloon makes us move up, however it is also true that by removing a weight we can also move up since the balloons can have a 'larger contribution'. Therefore, blank 3 is add and blank 4 is remove.
Similarly to move down on the number line, we can add a weight or remove a balloon so the remaining weight have a bigger influence in the downwards movement. Thus, blank 5 is weight and blank 6 is balloon