"Zone of Control" - abbreviated to "ZOC" or "ZoC" - is a concept common to Civilization games, restricting military unit movement close to units controlled by another power. It usually works like this: if a unit is next to a rival unit, it is not allowed to move to (but can direct an attack towards) any other tile that is also next to a rival unit unless one of your units is on it.
Movement in or out of one of your cities is the standard exception to the rule. Other exceptions depend on the game. In some games the presence of one of your non-military units will allow the move, but in C-evo it will not. In some, you can enter an unoccupied rival city ignoring ZOC, but in C-evo you cannot.