Keratin is the main protein inside the hair fiber. It is why a strand can bend, stretch, and return to shape.
When hair is exposed to high heat, the keratin structure can denature. That means the protein unfolds in a way that does not simply snap back because conditioner was applied later. Cosmetic routines can smooth the surface and reduce breakage, but truly denatured keratin grows out or gets cut.
This is the line between hair that feels dry and hair that is structurally damaged.