Platelets make the blood clot. Too few and you could potentially bleed to death, too many and you could be at risk of thrombosis or stroke as a result of clotting. Normal range is 150-450 thousand per mm3 so yours look towards the high end of normal range.
The condition of abnormally high platelet count is called thrombocytaemia and it is a myeloproliferative disorder (not sure if these are the correct spellings) and can be caused by an enlarged spleen. I'm mentioning this in case you want to look it up but DO NOT SELF DIAGNOSE as that can be very dangerous.
It is commonly controlled by dosage of Hydrea/Hydroxyurea, you may also be required to take aspirin daily, and if controlled and if you have regular check-ups it is asymptomatic, does not reduce life expentancy, and is not life threatening, in other words you have nothing to worry about.