μrngd is OpenWrt's micro non-physical true random number generator based on timing jitter.
Using the Jitter RNG core, the rngd provides an entropy source that feeds into the Linux /dev/random device if its entropy runs low. It updates the /dev/random entropy estimator such that the newly provided entropy unblocks /dev/random.
The seeding of /dev/random also ensures that /dev/urandom benefits from entropy. Especially during boot time, when the entropy of Linux is low, the Jitter RNGd provides a source of sufficient entropy.