Platelets are a special sort of blood cell that acts as a clotting agent to bind together and protect damaged blood cells. So if you cut yourself or break the skin, platelets rush to the site of the abrasion and group together to form a scab, which protects from infection.
A healthy platelet count is generally considered to be between 150,000 to 450,000 per microfilter of blood. A higher count than this means a person is suffering from a condition called Thrombocytosis.
There are two different types of this condition, the first, known as "Primary Thrombocytosis" is caused by abnormal cells in the bone marrow which leads to an increase in platelet production, the reason for which is unknown. "Secondary Thrombocytosis" on the other hand occurs as a result of another condition- for example cancer.
Often the condition is asymptomatic which can make it difficult to diagnose, however in extreme cases a high platelet count can lead to blood clots and even stroke.