This is the 4-byte checksum of the Kickstart ROM data as encoded in the ROM image itself. The correct checksum is stored at offset 0x3FFE8-0x3FFEB in a 256KB Kickstart image and offset 0x7FFE8-0x7FFEB in a 512KB image. In any case, it begins 24 bytes from the end of the ROM image.
The checksum is used during Kickstart initialization (in V37+) to verify the contents of the ROM. If the stored checksum does not match the one calculated during initialization, a red screen is presented, the power LED is flashed ten times, and initialization is restarted.
WARNING: Kickstarts smaller than 512KB (pre-2.0) do not verify the encoded checksum! (Some Amiga emulators, such as UAE, will validate the Kickstart checksum prior to starting the CPU.)
If the contents of the Kickstart ROM is altered, the encoded checksum must be altered to match. This can be done with SumKick by Martin Mares (part of the MKick package) for Kickstart ROM images, or with SumKick by Benjamin Fuller (part of the KickFont package) for Amiga 1000 Kickstart disks.
These tools should only be used to correct Kickstart checksums for images you have modified yourself. Please do not distribute modified Kickstart ROM images, as they only serve to further confuse the Amiga community regarding the validity of ROM images.
Also, please do not use SumKick in an attempt to 'correct' the checksum of a bad Kickstart ROM dump/extraction - while it will prevent Kickstart from detecting a problem during early initialization, it will only serve to make an otherwise bad/malfunctioning image appear to be "genuine" and thus create problems for others. Any successful dump (via EPROM reader or software dump) will pass the ROM's internal checksum verification routine without modification.