Switches are used within the Triggers section. They are basically variables that you can use to set values against.
For example, one of the players units enters an area on the map. This causes a trigger to fire setting a switch to a specific value. Later on in the mission you can check the switch for this value and if it is found then perform another action.
Click New... to create a new switch.
NOTE : There is a maximum limit of 16 switches.