This is a log of the signed app index downloaded from https://f-droid.org/repo/index.jar This is stored in a git repo, which serves as an imperfect append-only storage mechanism. People can then check that any file that they received from that F-Droid repository was a publicly released file.
It was gathered using a script in a crontab on a random Debian box on
the internet. Then once
fdroid btlog became available, the data was
since generated using
fdroid btlog on a more suitable random Debian
box on the internet.
For more info: