Raw egg "whites" are made of water and protein. The raw egg white is clear because the folded proteins are dissolved in the water, and they do not reflect light.

When they are heated, however, the proteins unfold. Unfolded proteins expose their many hydrophobic side chains:

The hydrophobic side chains from different unfolded proteins interact to form a large, solid mass. The molecules pack together tightly and reflect the light that formerly passed through, so the cooked egg white becomes solid and truly white.