Complex sentence: It is formed by the action of the protease thrombin on fibrinogen which is then polymerised and together with platelets forms a hemostatic plug or clot over a wound site.
Split into simpler sentences: It is formed by the action of the protease thrombin on fibrinogen which causes it to polymerize. The polymerized fibrin together with platelets forms a hemostatic plug or clot over a wound site.