It means if G is 1, H HAS to be in 2, but H can be in 2 without G being in 1.

Like wise, if H is in 1 G HAS to be 2, but G can be in 2 without H being in 1.

G and H can both be in 2. If one is in 1, the other HAS to be in 2. As a general rule, when you diagram it out, the necessary conditions(right of arrow) can occur without the sufficient condition(left of arrow) occurring.