The “Entry Burn” is the maneuver that SpaceX uses on the Falcon 9 booster immediately before atmospheric entry. It’s used to slow down the booster before it encounters the atmosphere; it’s typically used from about 70km down to 40km. If the reentry burn is skipped or shortened, the booster will break apart at about 40km due to the exponential increase in atmospheric pressure.

Here’s the approximate profile of a return-to-launch-site profile with the Falcon 9.