A spacecraft entering an atmosphere faces intense heating. An ablative heat shield protects the structure by letting its outer material char, decompose and erode in a controlled way. Energy goes into those changes and into material carried away from the surface.
A protective layer that is consumed
NASA’s PICA is one example of an ablative thermal protection material used on planetary entry missions. The shield also insulates what lies beneath while its surface is exposed to hot flow. This differs from trying to make a single metal surface withstand the entire heat load without changing.
Designed for a specific entry
A shield must be sized for the trajectory, speed, atmosphere and vehicle shape. Losing material is part of the design, but too much loss or uneven behavior can be dangerous. Ground tests and models help engineers predict performance for the expected entry conditions; one heat-shield recipe cannot be assumed suitable for every mission.
Related: how superalloys survive jet-engine heat.
Source
NASA Science: Protecting Future Planetary Missions from Extreme Heat.