In one well is the antiserum; in the other is the sample with unknown antigen composition. The two diffuse out of their respective wells toward each other. If the sample has an antigen that will react with the antibody, then a precipitation line will form at the region of optimal proportions as their diffusion paths pass.
If no precipitation line forms, the test is considered negative. That is, the sample has no antigen that will react with the antibodies.