The AVR Dx devices provide a Cyclic Redundancy Check mechanism, which is an important safety feature. By ensuring no code corruption has occurred, a potentially unintended behavior in the application that can cause a dangerous situation can be avoided.
The CRCSCAN peripheral scans the Nonvolatile Memory (NVM), making sure the code is not corrupted. It generates a checksum that is compared to a pre-calculated one. If the two checksums match, the Flash is OK, and the application code can start running.