HDBC Simplified - Formation of Platelets plug:

Formation of Platelets plug:    Tiny bits in your blood called platelets get "turned on" by triggers released when a blood vessel is damaged. They stick to the walls in the area and each other, changing shape to form a plug that fills in the broken part to stop blood from leaking out. When activated, platelets also release chemicals to attract more platelets and other cells, and to set off the next step.