This number is the git ID created when the release was committed. You can find it here: https://github.com/raspberrypi/firmware/releases
This release was incorporated to the Raspbian release you're using.
This ID is of little use, except to the people who publish Raspbian.
If you want to check if the OS image you downloaded is ok, run a command to compute the sha-256 signature of the file you downloaded. E.g. "sha256sum <raspbian_latest.img>" or "sha256sum <raspbian_latest.img.gz>"
One of these commands (I don't know if the signature is generated against the image or the compressed image) should output the same signature as the one published on the download page
, currently 6e3aa76e21473ef316c0bfc9efa5c27a27fe46bd698f71de3e06e66b64a55500
If the signatures match, this means the image you downloaded is an exact copy of what was published.
If they don't, check your commands, try to re-download as the file may have been corrupted during download. Also make sure your browser (or whatever you use for downloading) does not unpack or otherwise process the file automatically on arrival.