Blood is transformed into a solid gel when, at the site of vessel damage, plasma is converted to , which aggregates and forms a mesh.

Hemostasis is the process of the body that seals blood vessels that rupture. The process is basically starts with an injury, then vascular spasm, platelet plug formation and then coagulation. 

During blood clot formation, blood is transformed into solid gel at site of damage, where plasma fibrinogen is converted into loose fibrin molecules, which bind together to form mesh. Platelets and blood cells get trapped here by the fibrin strands, which produces a clot. This part of the clot formation is called coagulation. 

Very hard to understand