2.62 understand that platelets are involved in blood clotting, which prevents blood loss and the entry of micro-organisms

Platelets are small fragments of bone marrow. When you damage a blood vessel (e.g sever it if you cut yourself) the platelets in your blood will clump together at the damaged area, preventing any more blood from escaping and microorganisms getting into your body via the wound. This is known as blood clotting, and the clump of platelets is known as a blood clot.

NOTE: incase you were wondering, the platelets are held together by a mesh of a protein called fibrin.