This is a 2 channel loop detector and will detect a vehicle moving towards your gates and open the gates without you having to do anything. A 2 channel loop detector can have 2 loops attached to the detector, mainly for entry and exit loops.
They work by having a cable in the ground in a big loop, looping round many times (cable sold seperately). This is connected to the loop detector and as the car drives over the loop of cable it will be able to pick up a very small current generated by the metalic object passing above and will open the gates.
Have you every wondered how traffic lights seem to know when you have pulled up to them and they change, same thing here, in each lane there will be a loop of cable connected to a loop detector, when you drive over it, it will tell them to change the lights to green, mainly at night time or less busy periods.
These can be used to enter and exit your gates, however this solution offers no security so they are recommended for exit but for use with a timer clock so they can be switched off over night. This is so if somebody manages to steal your vehicle then they can't just drive towards your gates and they will open, with a timer clock they would have to have access to a fob or key code.