Blood contains red blood cells, white blood cells, plasma and platelets. When you get a cut, platelets (cell fragments that play an important role in forming blood clots) collect and stick to the edge of the cut in the blood vessel. Then the platelets release chemicals. The chemicals begin to produce some thread-like substances called "fibrin". Fibrin begins to form a net like substance across the opening of the cut in the blood vessel. This net traps and collects platelets and blood cells. This forms the blood clot that stops the bleeding. As the part of the clot that is on the surface dries out, it forms a scab.
In life science text books there is often a magnified picture showing fibrin and blood cells. This activity creates a hands-on model of that picture and illustrates the concept involved.