What is Dual Voltage in Electronics

If your mobile gadget is rated dual voltage then you do not require a transformer to convert the electrical current.
Your mobile gadget will operate on both 110/125v and 220/250v currents. You will only need adapter plugs.
Check the back of your mobile gear for a label that reads 100v/250v or says something similar to Wide Range Input. If your mobile gadget has a label like that, it will automatically adjust for the higher current. You may have to change it with a switch though. If you have to use a switch, remember to change the switch before getting connected.