This is the map/ part in campaign in which the Combine turn on you. In the map before this you start to move out of the city. During this, someone will give an order in which you will hear on your radio and see on a video screen. The order is telling the combine to arrest the civilian/ police force defending the cities or shoot to kill if not complying which in turn the military would take over.
In this mission you have to reach a rebel station outside of the city. You will be met by little resistance, but the further you get to the base, the more combine there is, since the base will be under attack. Once at the base you will have to defend it and then make your way to another town.