The Northern Lights or the Aurora Borealis, are caused by the interaction of particles from the sun with the upper atmosphere near the North Pole. If you are visiting Iceland during the winter months, from September to April you have a good chance of seeing them.
The Northern Lights tours usually depart around 8PM in the evening and the guide will take you out to the countryside away from any light pollution to get the best experience. You can choose between a group tour or private tour as well a bus tour, super jeep tour or a boat tour.
The Northern Lights are weather dependent and visibility can no be guaranteed. The guides will always check the weather forecast before leaving for the tour and if the tour is canceled you can usually reschedule for another night.