The root filesystem is a basic set of packages needed to provide a usable login environment around a kernel and kernel modules. The rootfs must be extensible to a full system so normal packaging tools need to be supported. See also Emdebian website.
The main Debian root filesystem is based on 'Essential' - a collection of packages that contain: Essential: yes in debian/control and although small in Debian terms, it is still much too large for Emdebian.
This page concentrates on how to use the emdebian-rootfs package to build an Emdebian root filesystem for your device and describes how to customize your package set for your needs.