The platelet count is the number of platelets in the blood. Highs and lows can both point to abnormal conditions of excess bleeding or clotting. I had high platelets with my iron deficiency anemia, very common.
Mean platelet volume (MPV) is a measurement (taken by a machine) that tells the average size of your platelets. New ones are larger, and an increased MPV happens when an increased amount of platelets are being made. This tells your doctor about the platelets that are being made in the bone marrow. Hope that helps a little.