What is reentry?
Also called: re-entry, atmospheric entry
Reentry is the return of a spacecraft into the atmosphere. The heat comes from the air being violently compressed in front of the vehicle, not from friction.
A returning spacecraft arrives extremely fast — the air ahead of it cannot get out of the way in time, so it is squashed. Compressing a gas heats it, and squashing it that hard makes it glow.
That is why a heat shield is blunt rather than pointed. A blunt shape pushes a shock wave out ahead of it and keeps the hottest gas at arm’s length from the structure.
A Falcon 9 booster does something similar with its entry burn: it fires engines against the airflow to slow down before the worst of the heating, rather than shielding itself against it.
See it happen
Coming HomeLanding on its feetSource: Falcon 9 Block 5 mission timeline (Starlink Group 17-38)