It is like haveged but for earlier boot. Here;s the author’s description:
haveged tries to use CPU jitter, in a way similar to jytter but
on a much more massive scale, to gather entropy-ish and writes
that to the kernel RNG. It, however, does that all the time, and
not just a little bit. Basically, it’s an attempt to gather entropy,
while early-rng-init-tools just takes what’s there during normal
system runtime (which you have to provide yourself, at the very
least before installing it, but sensibly also normally) and makes
it available to the kernel earlier (this really ought to be done
in the bootloader, even, but this at least improves on what we
currently have).