So if the blood has the corresponding antigen, in this case special proteins, it clots/clumps together. In this example we can see clots in B and AB, that's because the B-antibodies in both of those test chemicals are reacting to the B proteins expressed on the red blood cells. We know it's B- because the Rh antibody didn't react meaning the RH proteins are either not expressed or are completely absent.

If someone was O+, for example, since O blood has zero expression of A or B proteins on its surface you would not see any clotting except for the Rh protein.

Absolutely zero clotting if O-    (which is the universal donor -ie: can give blood to anyone = Gdad)

And you'd see clotting in every circle for AB+    (the universal recipient - ie: can receive blood from anyone = Gma)