The Mid Level Controller is based on the Raspberry Pi Zero W. It is part of the vehicle on-board electronics.
Its purposes are:
The setup and software build for the Raspberry Pi is explained under Vehicle Raspberry Pi.
A model predictive controller runs on-board on the Raspberry Pi. It can steer the vehicle to follow a given trajectory.
To see how to use it to control a vehicle, go to Vehicle Commands.
To see how it works, go to https://git.rwth-aachen.de/CPM/Project/Lab/software/blob/master/matlab_scripts/vehicle_dynamics_identification_and_mpc/documentation/main.pdf